Post-Effective Amendment to Registration Statement (Form POS AMI)

As Filed With the Securities and Exchange Commission on April 30, 2026

REGISTRATION NO. 811-08162

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

AMENDMENT NO. 155

MASTER INVESTMENT PORTFOLIO

(EXACT NAME OF REGISTRANT AS SPECIFIED IN CHARTER)

400 HOWARD STREET

SAN FRANCISCO, CALIFORNIA 94105

(ADDRESS OF PRINCIPAL EXECUTIVE OFFICES)

REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE: (800) 441-7762

JOHN M. PERLOWSKI

MASTER INVESTMENT PORTFOLIO

50 HUDSON YARDS

NEW YORK, NEW YORK 10001

(NAME AND ADDRESS OF AGENT FOR SERVICE)

COPIES TO:

MASTER INVESTMENT PORTFOLIO

U.S. TOTAL BOND INDEX MASTER PORTFOLIO

S&P 500 INDEX MASTER PORTFOLIO

EXPLANATORY NOTE

This is the combined Part A and Part B of the Registration Statement on Form N-1A for U.S. Total Bond Index Master Portfolio and S&P 500 Index Master Portfolio (each, a "Master Portfolio" and together, the "Master Portfolios"). Each Master Portfolio is a portfolio of Master Investment Portfolio ("MIP"), an open-end, series management investment company. U.S. Total Bond Index Master Portfolio is a diversified portfolio of MIP. S&P 500 Index Master Portfolio intends to be diversified in approximately the same proportion as its underlying index, the Standard & Poor's 500^® Index (the "S&P 500 Index" or the Master Portfolio's "Underlying Index"), is diversified. S&P 500 Index Master Portfolio is currently classified as a diversified portfolio under the Investment Company Act of 1940, as amended (the "1940 Act").

Each Master Portfolio operates as part of a master/feeder structure, and one or more corresponding feeder funds invest all of its/their assets in a Master Portfolio with substantially the same investment objective, strategies and policies as the corresponding feeder fund. Throughout this combined Part A and Part B for the Master Portfolios, specified information concerning the Master Portfolios and MIP is incorporated by reference from the most recently effective post-effective amendment to the registration statement on Form N-1A under the 1940 Act, of BlackRock Funds III (File Nos. 33-54126; 811-07332) (the "Trust") that relates to and includes the prospectuses and the statements of additional information of iShares U.S. Aggregate Bond Index Fund and iShares S&P 500 Index Fund (each, a "BlackRock Funds III Feeder Fund" and together, the "BlackRock Funds III Feeder Funds"), each a series of the Trust, a separate publicly offered investment company organized as a Delaware statutory trust. Each BlackRock Funds III Feeder Fund invests all of its assets in the corresponding Master Portfolio as follows: iShares U.S. Aggregate Bond Index Fund invests all of its assets in U.S. Total Bond Index Master Portfolio, and iShares S&P 500 Index Fund invests all of its assets in S&P 500 Index Master Portfolio. To the extent that information concerning a Master Portfolio and/or MIP is incorporated by reference and the Trust has filed, pursuant to Rule 497 under the Securities Act of 1933, as amended (the "1933 Act"), a supplement to a BlackRock Funds III Feeder Fund's prospectus or statement of additional information that supplements such incorporated information, then the supplemented information contained in such Rule 497 filing is also incorporated herein by reference. Each BlackRock Funds III Feeder Fund's current prospectuses and statement of additional information, as supplemented from time to time, are referred to herein collectively as the "Prospectus" and "SAI," respectively. Each feeder fund that invests in a Master Portfolio (including each BlackRock Funds III Feeder Fund) is referred to herein as a "Feeder Fund" and collectively as the "Feeder Funds." From time to time, a Master Portfolio may have one or more feeder funds that are not BlackRock Funds III Feeder Funds.

PART A - PROSPECTUS

April 30, 2026

THIS PART A DOES NOT CONSTITUTE AN OFFER TO SELL, OR THE SOLICITATION OF AN OFFER TO BUY, ANY "SECURITY" WITHIN THE MEANING OF THE 1933 ACT.

The Master Portfolios' Part B, dated April 30, 2026, is incorporated by reference into this Part A.

ITEMS 1 THROUGH 4.

Responses to Items 1 through 4 have been omitted pursuant to General Instruction B, Paragraph 2(b), to Form N-1A under the 1940 Act.

(a) INVESTMENT ADVISER

BlackRock Fund Advisors ("BFA" or the "Manager") is the investment adviser of each Master Portfolio.

(b) PORTFOLIO MANAGERS

Information concerning each Master Portfolio's portfolio managers is set forth below:

Interests in each Master Portfolio are issued solely in private placement transactions that do not involve any "public offering" within the meaning of Section 4(a)(2) of the 1933 Act. Investments in a Master Portfolio may only be made by investment companies or certain other entities that are "accredited investors" within the meaning of Regulation D under the 1933 Act.

The Master Portfolios have no minimum initial or subsequent investment requirements.

Each Feeder Fund may redeem all or any portion of its investment in the applicable Master Portfolio on any business day on which the New York Stock Exchange (the "NYSE") is open at the net asset value next determined after a redemption request is received in proper form by the Master Portfolio.

To the extent that a Master Portfolio has more than one Feeder Fund, it is treated as a non-publicly traded partnership for U.S. federal income tax purposes. To the extent that a Master Portfolio has only one Feeder Fund

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(either (i) directly or (ii) indirectly through an entity that is disregarded for U.S. federal income tax purposes), then the Master Portfolio's existence as an entity separate from that Feeder Fund is disregarded for U.S. federal income tax purposes. Whether the Master Portfolio is a partnership or disregarded as a separate entity it will generally not be subject to U.S. federal income tax.

Not applicable.

(a) INVESTMENT OBJECTIVE

U.S. Total Bond Index Master Portfolio

U.S. Total Bond Index Master Portfolio seeks to provide investment results that correspond to the total return performance of fixed-income securities in the aggregate, as represented by the Bloomberg U.S. Aggregate Bond Index (the "Bloomberg U.S. Aggregate Index" or the Master Portfolio's "Underlying Index").

S&P 500 Index Master Portfolio

S&P 500 Index Master Portfolio seeks to provide investment results that correspond to the total return performance of publicly-traded common stocks in the aggregate, as represented by the S&P 500 Index.

(b) IMPLEMENTATION OF INVESTMENT OBJECTIVES

U.S. Total Bond Index Master Portfolio

Under normal circumstances, at least 90% of the value of U.S. Total Bond Index Master Portfolio's assets, plus the amount of any borrowing for investment purposes, is invested in securities comprising the Bloomberg U.S. Aggregate Index, which, for the Master Portfolio, are considered bonds. The Master Portfolio will provide interestholders with at least 60 days' notice of any change to this policy. The Master Portfolio attempts to achieve, in both rising and falling markets, a correlation of at least 95% between the total return of its net assets before fees and expenses and the total return of the Master Portfolio's benchmark index, the Bloomberg U.S. Aggregate Index. Notwithstanding the factors described below, perfect (100%) correlation would be achieved if the total return of the Master Portfolio's net assets, before fees and expenses, increased or decreased exactly as the total return of the Master Portfolio's benchmark index increased or decreased. The Master Portfolio's ability to match its investment performance to the investment performance of its benchmark index may be affected by, among other things, the Master Portfolio's expenses, the amount of cash and cash equivalents held by the Master Portfolio, the manner in which the total return of the Master Portfolio's benchmark index is calculated, the size of the Master Portfolio's investment portfolio, and the timing, frequency and size of purchases of interests and redemptions.

The Master Portfolio utilizes sampling techniques that are designed to allow the Master Portfolio to duplicate substantially the investment performance of the Bloomberg U.S. Aggregate Index. However, the Master Portfolio is not expected to track the Bloomberg U.S. Aggregate Index with the same degree of accuracy that complete replication of the Bloomberg U.S. Aggregate Index would provide. No attempt is made to manage the Master Portfolio using economic, financial or market analysis. In addition, at times, the portfolio composition of the Master Portfolio may be altered (or "rebalanced") to reflect changes in the characteristics of the index that the Master Portfolio tracks.

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The Master Portfolio may invest in mortgage dollar rolls and participate in to-be-announced ("TBA") transactions on a regular basis to obtain exposure to mortgage-backed securities.

The Master Portfolio also may engage in futures and options transactions and other derivative securities transactions and lend its portfolio securities, each of which involves risk. The Master Portfolio may use futures contracts, options and other derivative transactions to manage its short-term liquidity and/or as substitutes for comparable market positions in the securities in its benchmark index. The Master Portfolio may also invest in high-quality money market instruments, including shares of money market funds advised by BFA or its affiliates.

Investors look to indexes as a standard of market performance. Indexes are model portfolios, that is, groups of stocks or bonds selected to represent an entire market or market segment. One way an index fund seeks to match an index's performance, before fees and expenses, is by buying and selling all of the index's securities in the same proportion as they are reflected in the index.

As of March 31, 2026, there were approximately 14,006 fixed-income securities included in the Bloomberg U.S. Aggregate Index. The Master Portfolio maintains a weighted average maturity consistent with that of the Bloomberg U.S. Aggregate Index, which generally ranges between 5 and 10 years. As a practical matter, it would be inefficient for the Master Portfolio to hold each security included in the Bloomberg U.S. Aggregate Index. The Master Portfolio can, however, substantially replicate the Bloomberg U.S. Aggregate Index's profile by holding a representative sample of securities in the Bloomberg U.S. Aggregate Index. It may, for example, hold U.S. Government obligations and corporate bonds in a similar proportion to the Bloomberg U.S. Aggregate Index. Additionally, it can match certain Bloomberg U.S. Aggregate Index features such as:

The Master Portfolio is designed for investors who desire a convenient way to invest in bonds issued in the United States. Although this market has increased in value over the long term, it fluctuates and has also decreased in value over shorter time periods.

The Master Portfolio does not by itself constitute a balanced investment program. Diversifying your investments by buying shares in other funds may improve your long-term return as well as reduce volatility.

The Bloomberg U.S. Aggregate Index is maintained by Bloomberg Index Services Limited ("Bloomberg"). Bloomberg does not sponsor, endorse, sell or promote the Feeder Funds or the Master Portfolio. Bloomberg makes no representation or warranty, expressed or implied, regarding the advisability of investing in a Feeder Fund or the Master Portfolio. Neither BFA nor BlackRock Advisors, LLC ("BAL") has or will have a role in maintaining the Bloomberg U.S. Aggregate Index.

The past performance of the Underlying Index is not a guide to future performance. BFA does not guarantee the accuracy or the completeness of the Underlying Index or any data included therein and BFA shall have no liability for any errors, omissions or interruptions therein. BFA makes no warranty, express or implied, to the owners of interests of the Master Portfolio or to any other person or entity, as to results to be obtained by the Master Portfolio from the use of the Underlying Index or any data included therein. Without limiting any of the foregoing, in no event shall BFA have any liability for any special, punitive, direct, indirect or consequential damages (including lost profits), even if notified of the possibility of such damages.

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S&P 500 Index Master Portfolio

Under normal circumstances, at least 90% of the value of S&P 500 Index Master Portfolio's assets, plus the amount of any borrowing for investment purposes, is invested in securities comprising the S&P 500 Index. The Master Portfolio will provide interestholders with at least 60 days' notice of any change to this policy. The Master Portfolio attempts to achieve, in both rising and falling markets, a correlation of at least 95% between the total return of its net assets before fees and expenses and the total return of the Master Portfolio's benchmark index, the S&P 500 Index. Notwithstanding the factors described below, perfect (100%) correlation would be achieved if the total return of the Master Portfolio's net assets, before fees and expenses, increased or decreased exactly as the total return of the Master Portfolio's benchmark index increased or decreased. The Master Portfolio's ability to match its investment performance to the investment performance of its benchmark index may be affected by, among other things, the Master Portfolio's expenses, the amount of cash and cash equivalents held by the Master Portfolio, the manner in which the total return of the Master Portfolio's benchmark index is calculated; the size of the Master Portfolio's investment portfolio; and the timing, frequency and size of purchases of interests and redemptions.

The Master Portfolio seeks to replicate the total return performance of the S&P 500 Index by investing the Master Portfolio's assets so that the percentage of assets of the Master Portfolio invested in a given stock is approximately the same as the percentage such stock represents in the S&P 500 Index. No attempt is made to manage the Master Portfolio using economic, financial or market analysis. In addition, at times, the portfolio composition of the Master Portfolio may be altered (or "rebalanced") to reflect changes in the characteristics of the index that the Master Portfolio tracks.

The Master Portfolio also may engage in futures and other derivative securities transactions and lend its portfolio securities, each of which involves risk. The Master Portfolio may use futures contracts and other derivative transactions to manage its short-term liquidity and/or as substitutes for comparable market positions in the securities in its benchmark index. The Master Portfolio may also invest in high-quality money market instruments, including shares of money market funds advised by BFA or its affiliates.

Investors look to indexes as a standard of market performance. Indexes are model portfolios, that is, groups of stocks or bonds selected to represent an entire market or market segment. One way an index fund seeks to match an index's performance, before fees and expenses, is by buying and selling all of the index's securities in the same proportion as they are reflected in the index. This is what the Master Portfolio does.

The Master Portfolio is designed for investors who desire a convenient way to invest in a broad spectrum of U.S. large cap stocks. Although this market has increased in value over the long term, it fluctuates and has also decreased in value over shorter time periods. This volatility is particularly characteristic of stocks.

The Master Portfolio does not by itself constitute a balanced investment program. Diversifying your investments by buying shares in other funds may improve your long-term return as well as reduce volatility.

S&P Dow Jones Indices LLC ("SPDJI") does not sponsor, endorse, sell or promote the Feeder Funds or the Master Portfolio, nor is it affiliated in any way with BAL, BFA, the Feeder Fund or the Master Portfolio. "Standard & Poor's^®," "S&P^®," and "S&P 500^®" are trademarks of Standard & Poor's Financial Services LLC (a division of S&P Global Inc.) licensed for use for certain purposes by BlackRock Institutional Trust Company, N.A. SPDJI makes no representation or warranty, express or implied, regarding the advisability of investing in a Feeder Fund or the Master Portfolio.

The past performance of the Underlying Index is not a guide to future performance. BFA does not guarantee the accuracy or the completeness of the Underlying Index or any data included therein and BFA shall have no liability for any errors, omissions or interruptions therein. BFA makes no warranty, express or implied, to the owners of interests of the Master Portfolio or to any other person or entity, as to results to be obtained by the Master Portfolio from the use of the Underlying Index or any data included therein. Without limiting any of the

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foregoing, in no event shall BFA have any liability for any special, punitive, direct, indirect or consequential damages (including lost profits), even if notified of the possibility of such damages.

The Master Portfolio intends to be diversified in approximately the same proportion as the Underlying Index is diversified. The Master Portfolio may become "non-diversified," as defined in the 1940 Act, solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Underlying Index. Interestholder approval will not be sought if the Master Portfolio becomes "non-diversified" due solely to a change in the relative market capitalization or index weighting of one or more constituents of the Underlying Index.

(c) RISKS

This section contains a discussion of the general risks of investing in each Master Portfolio. As with any fund, there can be no guarantee that a Master Portfolio will meet its investment objective or that a Master Portfolio's performance will be positive over any period of time. Investors may lose money investing in a Master Portfolio. An investment in a Master Portfolio is not a deposit in any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or by any bank or governmental agency. The order of the below risk factors does not indicate the significance of any particular risk factor.

Set forth below are the principal risk factors of investing in each Master Portfolio.

Credit Risk - Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer's credit rating or the market's perception of an issuer's creditworthiness may also affect the value of the Master Portfolio's investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.

Interest Rate Risk - The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise.

The Master Portfolio may be subject to a greater risk of rising interest rates during a period of historically low interest rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Master Portfolio's investments would be expected to decrease by 10%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Master Portfolio's investments will not affect interest income derived from instruments already owned by the Master Portfolio, but will be reflected in the Master Portfolio's net asset value. The Master Portfolio may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Master Portfolio management.

To the extent the Master Portfolio invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Master Portfolio) when interest rates rise. Moreover, because rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Master Portfolio to the extent that it invests in floating rate debt securities.

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These basic principles of bond prices also apply to U.S. Government securities. A security backed by the "full faith and credit" of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change.

Changing interest rates may have unpredictable effects on markets, may result in heightened market volatility, and could negatively impact the Master Portfolio's performance. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from mutual funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Master Portfolio to sell assets at inopportune times or at a loss or depressed value and could hurt the Master Portfolio's performance.

During periods of very low or negative interest rates, the Master Portfolio may be unable to maintain positive returns. Certain countries have recently experienced negative interest rates on certain fixed-income instruments. Very low or negative interest rates may magnify interest rate risk. Changing interest rates, including rates that fall below zero, may have unpredictable effects on markets, may result in heightened market volatility and may detract from Master Portfolio performance to the extent the Master Portfolio is exposed to such interest rates.

Extension Risk - When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall. Rising interest rates tend to extend the duration of securities, making them more sensitive to changes in interest rates. The value of longer-term securities generally changes more in response to changes in interest rates than shorter-term securities. As a result, in a period of rising interest rates, securities may exhibit additional volatility and may lose value.

Prepayment Risk - When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Master Portfolio may have to invest the proceeds in securities with lower yields. In periods of falling interest rates, the rate of prepayments tends to increase (as does price fluctuation) as borrowers are motivated to pay off debt and refinance at new lower rates. During such periods, reinvestment of the prepayment proceeds by the management team will generally be at lower rates of return than the return on the assets that were prepaid. Prepayment reduces the yield to maturity and the average life of the security.

Leverage Risk - The Master Portfolio's use of derivatives can magnify the Master Portfolio's gains and losses. Relatively small market movements may result in large changes in the value of a derivatives position and can result in losses that greatly exceed the amount originally invested.

Market Risk - Some derivatives are more sensitive to interest rate changes and market price fluctuations than other securities. The Master Portfolio could also suffer losses related to its derivatives positions as a result of unanticipated market movements, which losses are potentially unlimited. Finally, BFA may not be able to predict correctly the direction of securities prices, interest rates and other economic factors, which could cause the Master Portfolio's derivatives positions to lose value.

Counterparty Risk - Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will be unable or unwilling to fulfill its contractual obligation, and the related risks of having concentrated exposure to such a counterparty.

Illiquidity Risk -The possible lack of a liquid secondary market for derivatives and the resulting inability of the Master Portfolio to sell or otherwise close a derivatives position could expose the Master Portfolio to losses and could make derivatives more difficult for the Master Portfolio to value accurately.

Operational Risk - The use of derivatives includes the risk of potential operational issues, including documentation issues, settlement issues, systems failures, inadequate controls and human error.

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Legal Risk - The risk of insufficient documentation, insufficient capacity or authority of counterparty, or legality or enforceability of a contract.

Volatility and Correlation Risk - The Master Portfolio's use of derivatives may reduce the Master Portfolio's returns and/or increase volatility. Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Master Portfolio's use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.

Valuation Risk - Valuation for derivatives may not be readily available in the market. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them. Derivatives may also expose the Master Portfolio to greater risk and increase its costs. Certain transactions in derivatives involve substantial leverage risk and may expose the Master Portfolio to potential losses that exceed the amount originally invested by the Master Portfolio.

Hedging Risk - When a derivative is used as a hedge against a position that the Master Portfolio holds, any loss generated by the derivative generally should be substantially offset by gains on the hedged investment, and vice versa. While hedging can reduce or eliminate losses, it can also reduce or eliminate gains. Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Master Portfolio's hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences noted below.

Tax Risk - The federal income tax treatment of a derivative may not be as favorable as a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Master Portfolio realizes from its investments. As a result, a larger portion of the Master Portfolio's distributions may be treated as ordinary income rather than capital gains. In addition, certain derivatives are subject to mark-to-market or straddle provisions of the Internal Revenue Code of 1986, as amended (the "Internal Revenue Code"). If such provisions are applicable, there could be an increase (or decrease) in the amount of taxable dividends paid by a Feeder Fund. In addition, the tax treatment of certain derivatives, such as swaps, is unsettled and may be subject to future legislation, regulation or administrative pronouncements issued by the Internal Revenue Service.

Regulatory Risk - Derivative contracts are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (the "Dodd-Frank Act") in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, with respect to uncleared swaps, swap dealers are required to collect variation margin from the Master Portfolio and may be required by applicable regulations to collect initial margin from the Master Portfolio. Both initial and variation margin may be comprised of cash and/or securities, subject to applicable regulatory haircuts. Shares of investment companies (other than certain money market funds) may not be posted as collateral under applicable regulations. In addition, regulations adopted by global prudential regulators that are now in effect require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Master Portfolio, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Master Portfolio of trading in these instruments and, as a result, may affect returns to investors in the Master Portfolio.

Future regulatory developments may impact the Master Portfolio's ability to invest or remain invested in certain derivatives. Legislation or regulation may also change the way in which the Master Portfolio itself is regulated. BFA cannot predict the effects of any new governmental regulation that may be implemented on the ability of the Master Portfolio to use swaps or any other financial derivative product, and there can be no assurance that any new governmental regulation will not adversely affect the Master Portfolio's ability to achieve its investment objective.

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Risks Specific to Certain Derivatives Used by the Master Portfolio

Futures - Futures are standardized, exchange-traded contracts that obligate a purchaser to take delivery, and a seller to make delivery, of a specific amount of an asset at a specified future date at a specified price. The primary risks associated with the use of futures contracts and options are: (a) the imperfect correlations between the change in market value of the instruments held by the Master Portfolio and the price of the futures contract or option; (b) the possible lack of a liquid secondary market for a futures contract and the resulting inability to close a futures contract when desired; (c) losses caused by unanticipated market movements, which are potentially unlimited; (d) the investment adviser's inability to predict correctly the direction of securities prices, interest rates, currency exchange rates and other economic factors; and (e) the possibility that the counterparty will default in the performance of its obligations.

Options (U.S. Total Bond Index Master Portfolio) - An option is an agreement that, for a premium payment or fee, gives the option holder (the purchaser) the right but not the obligation to buy (a "call option") or sell (a "put option") the underlying asset (or settle for cash in an amount based on an underlying asset, rate, or index) at a specified price (the "exercise price") during a period of time or on a specified date. Investments in options are considered speculative. When the Master Portfolio purchases an option, it may lose the total premium paid for it if the price of the underlying security or other assets decreased, remained the same or failed to increase to a level at or beyond the exercise price (in the case of a call option) or increased, remained the same or failed to decrease to a level at or below the exercise price (in the case of a put option). If a put or call option purchased by the Master Portfolio were permitted to expire without being sold or exercised, its premium would represent a loss to the Master Portfolio. To the extent that the Master Portfolio writes or sells an option, if the decline or increase in the underlying asset is significantly below or above the exercise price of the written option, the Master Portfolio could experience a substantial loss.

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The index provider may rely on various sources of information to assess the criteria of components of the Underlying Index, including information that may be based on assumptions and estimates. Neither the Master Portfolio nor BFA can offer assurances that the index provider's methodology or sources of information will provide an accurate assessment of included components. Errors in index data, index computations or the construction of the Underlying Index in accordance with its methodology may occur, and the index provider may not identify or correct them promptly or at all, particularly for indexes that are less commonly used as benchmarks. Errors related to the Underlying Index may negatively or positively impact the Master Portfolio and its interestholders. For example, if the Underlying Index contains incorrect constituents, the Master Portfolio will have exposure to such constituents and will be underexposed to the Underlying Index's other constituents. Interestholders should understand that any gains from the index provider's errors will be kept by the Master Portfolio and its interestholders and any losses or costs from such errors will be borne by the Master Portfolio and its interestholders.

Unusual market conditions or other unforeseen circumstances (such as natural disasters, political unrest or war) may impact the index provider or a third-party data provider and could cause the index provider to postpone a scheduled rebalance to the Underlying Index. This could cause the Underlying Index to vary from its normal or expected composition. If a scheduled rebalance is postponed, index constituents that would otherwise be removed at the rebalance (due to, for example, changes in market capitalization or issuer credit ratings) may remain, causing the performance and constituents of the Underlying Index to vary from those expected under normal conditions. In addition, to the extent circumstances evolve between periodic index reviews and reconstitutions, the Underlying Index may include constituents that do not align with its objective or selection criteria, and the Master Portfolio tracking the Underlying Index may be similarly affected.

In addition to scheduled rebalances, the index provider or its agents may carry out ad hoc index rebalances due to reaching certain weighting constraints, unusual market conditions, corporate events, or corrections of errors. The Master Portfolio will in turn rebalance its portfolio to attempt to increase the correlation between the portfolio and the Underlying Index. The Master Portfolio and its interestholders will directly bear any transaction costs and market exposure from such portfolio rebalancing. Therefore, index related errors and ad hoc rebalances may increase the Master Portfolio's costs and tracking error.

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Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. The Master Portfolio's investments in asset-backed securities are subject to risks similar to those associated with mortgage-related securities, as well as additional risks associated with the nature of the assets and the servicing of those assets. These securities also are subject to the risk of default on the underlying mortgages or assets, particularly during periods of economic downturn. Certain CMBS are issued in several classes with different levels of yield and credit protection. The Master Portfolio's investments in CMBS with several classes may be in the lower classes that have greater risks than the higher classes, including greater interest rate, credit and prepayment risks.

Mortgage-backed securities may be either pass-through securities or collateralized mortgage obligations ("CMOs"). Pass-through securities represent a right to receive principal and interest payments collected on a pool of mortgages, which are passed through to security holders. CMOs are created by dividing the principal and interest payments collected on a pool of mortgages into several revenue streams ("tranches") with different priority rights to portions of the underlying mortgage payments. Certain CMO tranches may represent a right to receive interest only ("IOs"), principal only ("POs") or an amount that remains after floating-rate tranches are paid (an "inverse floater"). These securities are frequently referred to as "mortgage derivatives" and may be extremely sensitive to changes in interest rates. Interest rates on inverse floaters, for example, vary inversely with a short-term floating rate (which may be reset periodically). Interest rates on inverse floaters will decrease when short-term rates increase, and will increase when short-term rates decrease. These securities have the effect of providing a degree of investment leverage. In response to changes in market interest rates or other market conditions, the value of an inverse floater may increase or decrease at a multiple of the increase or decrease in the value of the underlying securities. If the Master

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Portfolio invests in CMO tranches (including CMO tranches issued by government agencies) and interest rates move in a manner not anticipated by Master Portfolio management, it is possible that the Master Portfolio could lose all or substantially all of its investment. TBA commitments are forward agreements for the purchase or sale of securities, including mortgage-backed securities for a fixed price, with payment and delivery on an agreed upon future settlement date. The specific securities to be delivered are not identified at the trade date. However, delivered securities must meet specified terms, including issuer, rate and mortgage terms. Certain mortgage-backed securities in which the Master Portfolio may invest may also provide a degree of investment leverage, which could cause the Master Portfolio to lose all or substantially all of its investment.

The mortgage market in the United States has experienced difficulties that may adversely affect the performance and market value of certain of the Master Portfolio's mortgage-related investments. Delinquencies and losses on mortgage loans (including subprime and second-lien mortgage loans) and a decline in or flattening of real estate values (in each case as has been experienced and may continue to be experienced in many housing markets) may exacerbate such delinquencies and losses. Also, a number of mortgage loan originators have experienced serious financial difficulties or bankruptcy. Reduced investor demand for mortgage loans and mortgage-related securities and increased investor yield requirements have caused limited liquidity in the secondary market for mortgage-related securities, which can adversely affect the market value of mortgage-related securities. It is possible that such limited liquidity in such secondary markets could continue or worsen.

Asset-backed securities entail certain risks not presented by mortgage-backed securities, including the risk that in certain states it may be difficult to perfect the liens securing the collateral backing certain asset-backed securities. In addition, certain asset-backed securities are based on loans that are unsecured, which means that there is no collateral to seize if the underlying borrower defaults.

Cybersecurity incidents can result from deliberate attacks or unintentional events against an issuer in which the Master Portfolio invests, the Master Portfolio or any of its service providers. They include, but are not limited to, gaining unauthorized access to systems, misappropriating assets or sensitive information, corrupting or destroying data, and causing operational disruption. Geopolitical tension may increase the scale and sophistication of deliberate attacks, particularly those from nation states or from entities with nation state backing. Cybersecurity incidents may result in any of the following: financial losses; interference with the Master Portfolio's ability to calculate its NAV; disclosure of confidential information;

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impediments to trading; submission of erroneous trades by the Master Portfolio or erroneous subscription or redemption orders; the inability of the Master Portfolio or its service providers to transact business; violations of applicable privacy and other laws; regulatory fines; penalties; reputational damage; reimbursement or other compensation costs; and other legal and compliance expenses. Furthermore, cybersecurity incidents may render records of the Master Portfolio, including records relating to its assets and transactions, interestholder ownership of Master Portfolio interests, and other data integral to the Master Portfolio's functioning, inaccessible, inaccurate or incomplete. Power outages, natural disasters, equipment malfunctions and processing errors that threaten information and technology systems relied upon by the Master Portfolio or its service providers, as well as market events that occur at a pace that overloads these systems, may also disrupt business operations or impact critical data. In addition, the risks of increased use of AI technologies, such as machine learning, include data risk, transparency risk, and operational risk. The AI technologies, which are generally highly reliant on the collection and analysis of large amounts of data, may incorporate biased or inaccurate data, and it is not possible or practicable to incorporate all relevant data into such technologies. The output or results of any such AI technologies may therefore be incomplete, erroneous, distorted or misleading. Further, AI tools may lack transparency as to how data is utilized and how outputs are generated. AI technologies may also allow the unintended introduction of vulnerabilities into infrastructures and applications. The Master Portfolio and its interestholders could be negatively impacted as a result of these risks associated with AI technologies. AI technologies and their current and potential future applications, and the regulatory frameworks within which they operate, continue to quickly evolve, and it is impossible to anticipate the full scope of future AI capabilities or rules and the associated risks to the Master Portfolio. While the Master Portfolio's service providers are required to have appropriate operational, information security and cybersecurity risk management policies and procedures, their methods of risk management may differ from those of the Master Portfolio in the setting of priorities, the personnel and resources available or the effectiveness of relevant controls. The Master Portfolio and its adviser seek to reduce these risks through controls, procedures and oversight, including establishing business continuity plans and risk management systems. However, there are inherent limitations in such plans and systems, including the possibility that certain risks that may affect the Master Portfolio have not been identified or may emerge in the future; that such plans and systems may not completely eliminate the occurrence or mitigate the effects of operational or information security disruptions or failures or of cybersecurity incidents; or that prevention and remediation efforts will not be successful or that incidents will go undetected. The Master Portfolio cannot control the systems, information security or other cybersecurity of the issuers in which it invests or its service providers, counterparties, and other third parties whose activities affect the Master Portfolio.

Lastly, the regulatory climate governing cybersecurity and data protection is developing quickly and may vary considerably across jurisdictions. Regulators continue to develop new rules and standards related to cybersecurity and data protection. Compliance with evolving regulations can be demanding and costly, requiring substantial resources to monitor and implement required changes.

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The United States has developed increasingly strained relations with a number of foreign countries. If relations with certain countries deteriorate, it could adversely affect U.S. issuers as well as non-U.S. issuers that rely on the United States for trade. The United States has also experienced increased internal political discord, as well as significant challenges in managing and containing the outbreak of COVID-19. If these trends were to continue, it may have an adverse impact on the U.S. economy and the issuers in which the Master Portfolio invests.

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Each Master Portfolio may also be subject to certain other non-principal risks associated with its investments and investment strategies, including:

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For example, ownership limits may apply to securities whose issuers operate in certain regulated industries or in certain international markets. Such limits also may apply where the investing entity (such as the Master Portfolio) is subject to corporate or regulatory ownership restrictions or invests in certain futures or other derivative transactions. In certain circumstances, aggregate and/or fund-level amounts invested or voted by BFA and its affiliates for client funds and accounts managed by BFA (including the Master Portfolio) may not exceed the relevant limits without the grant of a license or other regulatory or corporate approval, order, consent, relief or non-disapproval. However, there is no guarantee that permission will be granted, or that, once granted, it will not be modified or revoked at a later date with minimal or no notice. In other cases, exceeding such thresholds may cause BFA and its affiliates, the Master Portfolio or other client accounts to suffer disadvantages or business restrictions.

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(d) PORTFOLIO HOLDINGS

A description of the Master Portfolios' policies and procedures with respect to the disclosure of the Master Portfolios' portfolio holdings is available in Part B of this Registration Statement and is available free of charge by calling 1-800-441-7762 (toll-free).

(a)(1) MANAGEMENT

BFA serves as investment adviser to each Master Portfolio. BFA manages the investment of each Master Portfolio's assets and provides each Master Portfolio with investment guidance and policy direction in connection with daily portfolio management, subject to the supervision of MIP's board of trustees (the "Board of Trustees" or the "Board" and the members thereof, the "Trustees") and in conformity with Delaware law and the stated policies of the Master Portfolios.

BFA is an indirect majority-owned subsidiary of BlackRock, Inc. As of March 31, 2026, BFA and its affiliates (collectively, "BlackRock") had approximately $13.9 trillion in investment company and other portfolio assets under management.

MIP has entered into an investment management agreement with BFA, on behalf of each Master Portfolio (the "Management Agreement").

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With respect to U.S. Total Bond Index Master Portfolio, BFA receives as compensation for its services to U.S. Total Bond Index Master Portfolio a management fee equal to 0.04% of U.S. Total Bond Index Master Portfolio's average daily net assets.

With respect to S&P 500 Index Master Portfolio, effective February 24, 2026, BFA receives as compensation for its services to S&P 500 Index Master Portfolio a management fee equal to 0.007% of S&P 500 Index Master Portfolio's average daily net assets. Prior to February 24, 2026, BFA received as compensation for its services to S&P 500 Index Master Portfolio a management fee equal to 0.01% of S&P 500 Index Master Portfolio's average daily net assets.

BFA has contractually agreed to waive the management fee with respect to any portion of each Master Portfolio's assets estimated to be attributable to investments in other equity and fixed-income mutual funds and exchange-traded funds managed by BFA or its affiliates that have a contractual management fee, through June 30, 2027. In addition, BFA has contractually agreed to waive its management fees by the amount of investment advisory fees each Master Portfolio pays to BFA indirectly through its investment in affiliated money market funds. The contractual waiver with respect to each Master Portfolio is in effect through June 30, 2027. The contractual agreements may be terminated upon 90 days' notice by a majority of the non-interested trustees (the "Independent Trustees") of MIP or by a vote of a majority of the outstanding voting securities of the applicable Master Portfolio.

The fees and expenses of the Independent Trustees of MIP, counsel to the Independent Trustees of MIP and the independent registered public accounting firm that provides audit services in connection with the Master Portfolio (collectively referred to as the "MIP Independent Expenses") are paid directly by the Master Portfolio. Each of BAL and BFA, as applicable, has contractually undertaken to reimburse or provide an offsetting credit to the Master Portfolio for such MIP Independent Expenses through June 30, 2027. Each such contractual arrangement may not be terminated prior to July 1, 2027 unless approved by a majority of the Independent Trustees of MIP (with 90 days' notice) or by a vote of the majority of outstanding voting securities of the applicable Master Portfolio.

For the fiscal year ended December 31, 2025, BFA received a management fee, net of management fee waivers, as a percentage of the average daily net assets of each Master Portfolio as follows:

A discussion of the basis for approval by the Board of Trustees of the Management Agreement with BFA with respect to each Master Portfolio is available in the applicable BlackRock Funds III Feeder Fund's reports filed on Form N-CSR for the period ended June 30, 2025.

From time to time, a manager, analyst, or other employee of BFA or its affiliates may express views regarding a particular asset class, company, security, industry, or market sector. The views expressed by any such person are the views of only that individual as of the time expressed and do not necessarily represent the views of BFA or any other person within the BFA organization. Any such views are subject to change at any time based upon market or other conditions and BFA disclaims any responsibility to update such views. These views may not be relied on as investment advice and, because investment decisions for each Master Portfolio are based on numerous factors, may not be relied on as an indication of trading intent on behalf of the Master Portfolios.

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CONFLICTS OF INTEREST

The investment activities of BFA and its affiliates (including BlackRock, Inc. and its subsidiaries (collectively, the "Affiliates")), and their respective directors, officers or employees, in managing their own accounts and other accounts, may present conflicts of interest that could disadvantage the Master Portfolios and their interestholders.

BFA and its Affiliates are involved worldwide with a broad spectrum of financial services and asset management activities and in the ordinary course of business may engage in activities in which their interests or the interests of other clients may conflict with those of the Master Portfolios. BFA and its Affiliates act, or may act, as an investor, research provider, investment manager, commodity pool operator, commodity trading advisor, financier, underwriter, adviser, trader, lender, index provider, agent and/or principal. BFA and its Affiliates may have other direct and indirect interests in securities, currencies, commodities, derivatives and other assets in which the Master Portfolios may directly or indirectly invest.

BFA and its Affiliates may engage in proprietary trading and advise accounts and other funds that have investment objectives similar to those of the Master Portfolios and/or that engage in and compete for transactions in the same or similar types of securities, currencies and other assets as are held by the Master Portfolios. This may include transactions in securities issued by other open-end and closed-end investment companies, including investment companies that are affiliated with the Master Portfolios and BFA, to the extent permitted under the 1940 Act. The trading activities of BFA and its Affiliates are carried out without reference to positions held directly or indirectly by the Master Portfolios. These activities may result in BFA or an Affiliate having positions in assets that are senior or junior to, or that have interests different from or adverse to, the assets held by the Master Portfolios.

The Master Portfolios may invest in securities issued by, or engage in other transactions with, entities with which an Affiliate has significant debt or equity investments or other interests. The Master Portfolios may also invest in issuances (such as debt offerings or structured notes) for which an Affiliate is compensated for providing advisory, cash management or other services. The Master Portfolios also may invest in securities of, or engage in other transactions with, entities for which an Affiliate provides or may provide research coverage or other analysis.

An Affiliate may have business relationships with, and receive compensation from, distributors, consultants or others who recommend a Master Portfolio or who engage in transactions with or for a Master Portfolio.

Neither BFA nor any Affiliate is under any obligation to share any investment opportunity, idea or strategy with the Master Portfolios. As a result, an Affiliate may compete with the Master Portfolios for appropriate investment opportunities. The results of a Master Portfolio's investment activities, therefore, may differ from those of an Affiliate and of other accounts managed by an Affiliate. It is possible that a Master Portfolio could sustain losses during periods in which one or more Affiliates and other accounts achieve profits on their trading for proprietary or other accounts. The opposite result is also possible.

In addition, the Master Portfolios may enter into transactions in which BFA or an Affiliate or their directors, officers, employees or clients have an adverse interest. The Master Portfolios may be adversely impacted by the effects of transactions undertaken by BFA or an Affiliate or their directors, officers, employees or clients.

From time to time, BFA or its advisory clients (including other funds and accounts) may, subject to compliance with applicable law, purchase and hold interests of the Master Portfolios. The price, availability, liquidity, and (in some cases) expense ratio of the Master Portfolios may be impacted by purchases and sales of the Master Portfolios by BFA or its advisory clients.

The Master Portfolios' activities may be limited because of regulatory restrictions applicable to BFA or an Affiliate or their policies designed to comply with such restrictions.

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Under a securities lending program approved by the Board, MIP, on behalf of each Master Portfolio, has retained BlackRock Institutional Trust Company, N.A. ("BTC"), an Affiliate of BFA, to serve as the securities lending agent for the Master Portfolios to the extent that the Master Portfolios participate in the securities lending program. For these services, the securities lending agent will receive a fee from the participating Master Portfolio based on the returns earned on the Master Portfolio's lending activities, including the investment of the cash received as collateral for the loaned securities. In addition, one or more Affiliates may be among the entities to which the Master Portfolios may lend their portfolio securities under the securities lending program.

BFA and its Affiliates may benefit from a fund using a BlackRock index by creating increasing acceptance in the marketplace for such indexes. BFA and its Affiliates are not obligated to license an index to a fund, and no fund is under an obligation to use a BlackRock index. The terms of a fund's index licensing agreement with BFA or its Affiliates may not be as favorable as the terms offered to other licensees.

The activities of BFA and its Affiliates and their respective directors, officers or employees, may give rise to other conflicts of interest that could disadvantage the Master Portfolios and their interestholders. BFA has adopted policies and procedures designed to address these potential conflicts of interest. Please see the SAI for further information.

ANTI-MONEY LAUNDERING REQUIREMENTS

The Master Portfolios are subject to the USA PATRIOT Act (the "Patriot Act"). The Patriot Act is intended to prevent the use of the U.S. financial system in furtherance of money laundering, terrorism or other illicit activities. Pursuant to requirements under the Patriot Act, a Master Portfolio is required to obtain sufficient information from interestholders to enable it to form a reasonable belief that it knows the true identity of its interestholders. This information will be used to verify the identity of investors or, in some cases, the status of financial intermediaries. Such information may be verified using third-party sources. This information will be used only for compliance with the Patriot Act or other applicable laws, regulations and rules in connection with money laundering, terrorism or economic sanctions.

The Master Portfolios reserve the right to reject purchase orders from persons who have not submitted information sufficient to allow the Master Portfolios to verify their identity. The Master Portfolios also reserve the right to redeem any amounts in a Master Portfolio from persons whose identity it is unable to verify on a timely basis. It is the Master Portfolios' policy to cooperate fully with appropriate regulators in any investigations conducted with respect to potential money laundering, terrorism or other illicit activities.

BLACKROCK PRIVACY PRINCIPLES

BlackRock is committed to maintaining the privacy of its current and former fund investors and individual clients (collectively, "Clients") and to safeguarding their non-public personal information. The following information is provided to help you understand what personal information BlackRock collects, how we protect that information and why in certain cases we share such information with select parties.

If you are located in a jurisdiction where specific laws, rules or regulations require BlackRock to provide you with additional or different privacy-related rights beyond what is set forth below, then BlackRock will comply with those specific laws, rules or regulations.

BlackRock obtains or verifies personal non-public information from and about you from different sources, including the following: (i) information we receive from you or, if applicable, your financial intermediary, on applications, forms or other documents; (ii) information about your transactions with us, our affiliates, or others; (iii) information we receive from a consumer reporting agency; and (iv) from visits to our website.

BlackRock does not sell or disclose to non-affiliated third parties any non-public personal information about its Clients, except as permitted by law, or as is necessary to respond to regulatory requests or to service Client accounts. These non-affiliated third parties are required to protect the confidentiality and security of this information and to use it only for its intended purpose.

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We may share information with our affiliates to service your account or to provide you with information about other BlackRock products or services that may be of interest to you. In addition, BlackRock restricts access to non-public personal information about its Clients to those BlackRock employees with a legitimate business need for the information. BlackRock maintains physical, electronic and procedural safeguards that are designed to protect the non-public personal information of its Clients, including procedures relating to the proper storage and disposal of such information.

(a)(2) PORTFOLIO MANAGERS

Information concerning each Master Portfolio's portfolio managers is set forth below:

U.S. Total Bond Index Master Portfolio

S&P 500 Index Master Portfolio

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(b) ORGANIZATION AND CAPITAL STRUCTURE

MIP was organized on October 20, 1993 as a statutory trust under the laws of the State of Delaware and is registered as an open-end, series management investment company under the 1940 Act. MIP is a "series fund," which is a mutual fund company that has been divided into separate portfolios. Each portfolio is treated as a separate entity for certain matters under the 1940 Act and for certain other purposes. A holder of beneficial interests (an "interestholder") of a portfolio is not deemed to be an interestholder of any other portfolio of MIP. The Board of Trustees has authorized MIP to issue multiple series. MIP currently offers interests in the following series: Diversified Equity Alpha Master Portfolio, International Tilts Master Portfolio, Large Cap Index Master Portfolio, Money Market Master Portfolio, S&P 500 Index Master Portfolio, Total International ex U.S. Index Master Portfolio, Treasury Money Market Master Portfolio and U.S. Total Bond Index Master Portfolio. Information about the listed portfolios that is not covered in this combined Part A and Part B is contained in separate offering documents. From time to time, additional portfolios may be established and sold pursuant to separate offering documents.

All consideration received by MIP for interests in one of its portfolios and all assets in which such consideration is invested will belong to that portfolio (subject only to the rights of creditors of MIP) and will be subject to the liabilities related thereto. The income attributable to, and the expenses of, one portfolio are treated separately from those of each other portfolio.

The business and affairs of MIP are managed under the direction of its Board of Trustees. The office of MIP is located at 400 Howard Street, San Francisco, California 94105.

Please see Item 22 of Part B for a further description of MIP's capital structure.

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PURCHASE, REDEMPTION AND PRICING OF INTERESTS

Investments in a Master Portfolio are valued based on an interestholder's proportionate ownership interest (rounded to the nearest hundredth of a percent, although each Master Portfolio reserves the right to calculate proportionate ownership interests to more than two decimal places) in the Master Portfolio's aggregate net assets ("Net Assets") (i.e., the value of its total assets (including the securities held by the Master Portfolio plus any cash or other assets, including interest and dividends accrued but not yet received) less total liabilities (including accrued expenses)) as next determined after an order is received in proper form by the applicable Master Portfolio. The value of each Master Portfolio's Net Assets is determined as of the close of regular trading on the NYSE, which is generally 4:00 p.m. (Eastern time) ("Valuation Time") on each day the NYSE is open for business (a "Business Day"). If the NYSE closes early, the time for calculating each Master Portfolio's net asset value and the deadline for additions to or reductions in investments in the Master Portfolio will be accelerated to the earlier closing time.

An investor in a Master Portfolio may add to or reduce its investment in the Master Portfolio on any Business Day. At the Valuation Time on each Business Day, the value of each investor's beneficial interest in a Master Portfolio is determined by multiplying the Master Portfolio's Net Assets by the percentage, effective for that day, of that investor's share of the aggregate beneficial interests in the Master Portfolio. Any additions to or redemptions of those interests that are to be effected on that day will then be effected. Each investor's share of the aggregate beneficial interests in the Master Portfolio will then be recomputed using the percentage equal to the fraction: (i) the numerator of which is the value of the investor's cumulative investment in the Master Portfolio up to that day, plus or minus, as the case may be, the amounts of net additions or redemptions from such investment effected on that day and (ii) the denominator of which is the Master Portfolio's Net Assets as of the Valuation Time on that day, plus or minus, as the case may be, the amount of the net additions to or redemptions from the aggregate investments in the Master Portfolio by all investors. The percentages so determined are then applied to determine the value of each investor's respective interest in the Master Portfolio as of the Valuation Time on the following Business Day.

The value of the securities and other assets and liabilities held by the Master Portfolios are determined pursuant to BFA's valuation policies and procedures. BFA has been designated by the Board as the valuation designee for the Master Portfolio pursuant to Rule 2a-5 under the 1940 Act.

The Master Portfolios' investments are generally valued using market valuations. In the event that current market valuations are not readily available or such valuations do not reflect current market values, BFA will fair value the Master Portfolios' investments in accordance with its policies and procedures. The frequency with which a Master Portfolio's investments are valued using fair value pricing is primarily a function of the types of securities and other assets in which the Master Portfolio invests pursuant to its investment objective, strategies and limitations.

Investments that may be valued using fair value pricing include, but are not limited to: (i) an unlisted security related to corporate actions; (ii) a restricted security (i.e., one that may not be publicly sold without registration under the 1933 Act); (iii) a security whose trading has been suspended or which has been de-listed from its primary trading exchange; (iv) a security that is thinly traded; (v) a security in default or bankruptcy proceedings for which there is no current market quotation; (vi) a security or other asset affected by currency controls or restrictions; and (vii) a security affected by a significant event (i.e., an event that occurs after the close of the markets on which the security is traded but before the time as of which a Master Portfolio's net asset value is computed and that may materially affect the value of the Master Portfolio's investments). Examples of events that may be "significant events" are government actions, natural disasters, armed conflict, acts of terrorism, and significant market fluctuations.

Valuing a Master Portfolio's investments using fair value pricing will result in using prices for those investments that may differ from current market valuations.

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Because foreign markets may be open on different days than the days during which an interestholder may purchase a Master Portfolio's interests, the value of the Master Portfolio's investments may change on days when interestholders are not able to purchase the Master Portfolio's interests.

An investor in a Master Portfolio may redeem all or any portion of its interest on any Business Day at the net asset value next determined after a redemption request is received in proper form. The Master Portfolios generally remit the proceeds from a redemption the next Business Day after receiving a properly executed redemption order and no longer than seven days after receiving the order. MIP may, however, suspend the right of redemption or postpone redemption payments for longer than seven days for any period during which (i) the NYSE is closed (other than customary weekend and holiday closings); (ii) trading on the NYSE is restricted; (iii) an emergency exists as a result of which disposal or valuation of a Master Portfolio's investments is not reasonably practicable; or (iv) for such other periods as the SEC by order may permit, as permitted under Section 22(e) of the 1940 Act, and other applicable laws. In addition, the Master Portfolios reserve the right to refuse any purchase of interests. Investments in the Master Portfolios may not be transferred.

NET INVESTMENT INCOME AND CAPITAL GAIN ALLOCATIONS AND DISTRIBUTIONS

Any net investment income of the Master Portfolios generally will be accrued and allocated daily to all investors of record as of the Valuation Time on any Business Day. The Master Portfolios' net investment income for a Saturday, Sunday or holiday will be accrued and allocated to investors of record as of the Valuation Time on the previous Business Day. Allocations of a Master Portfolio's net investment income will be distributed to an interestholder's account on the applicable payment date. Capital gains realized by a Master Portfolio will be allocated on the date of accrual and distributed at least annually to an interestholder's account on the applicable payment date.

Each Master Portfolio will allocate its investment income, expenses, and realized and unrealized net gains and losses to its interestholders pro rata in accordance with their beneficial interests. Allocations of taxable income or loss may be made in a different manner in order to comply with U.S. federal income tax rules.

FREQUENT PURCHASES AND REDEMPTIONS OF INTERESTS

MIP does not offer its interests for sale to the general public, nor does it offer an exchange privilege. MIP is not, therefore, directly subject to the risks of short-term trading and the Board of Trustees has not adopted procedures to prevent such trading. However, MIP may be adversely affected by short-term trading in shares of a Feeder Fund. See "Account Information - Short-Term Trading Policy" in Part A of each BlackRock Funds III Feeder Fund's Registration Statement for more information.

TAXES

To the extent that a Master Portfolio has more than one Feeder Fund, it is treated as a non-publicly traded partnership for U.S. federal income tax purposes. To the extent that a Master Portfolio has only one Feeder Fund (either (i) directly or (ii) indirectly through an entity that is disregarded for U.S. federal income tax purposes), then the Master Portfolio is disregarded as an entity separate from that Feeder Fund.

Whether a Master Portfolio is a partnership or disregarded as a separate entity, it will generally not be subject to any U.S. federal income tax. Each of the Master Portfolio's Feeder Funds will take into account its allocable share of the applicable Master Portfolio's ordinary income, capital gains, losses, deductions and credits in determining its income tax liability and, if the Feeder Fund is intended to qualify as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code, the Feeder Fund's share of the Master Portfolio's income and assets.

It is intended that each Master Portfolio's assets, income and distributions will be managed in such a way that each Feeder Fund will be able to satisfy the requirements of Subchapter M of the Internal Revenue Code for

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qualification as a RIC, assuming that the Feeder Fund invests all of its investable assets in the applicable Master Portfolio and the Feeder Fund meets all other requirements for such qualification not within the control of the Master Portfolio.

Beneficial interests in each Master Portfolio are not registered under the 1933 Act because such interests are issued solely in transactions that are exempt from registration under the 1933 Act. Each Master Portfolio is a "master" in a "master/feeder" structure. Only "feeder funds" - i.e., investment companies that are "accredited investors" and invest all of their assets in the Master Portfolios - or certain other entities that are "accredited investors" within the meaning of Regulation D under the 1933 Act may make direct investments in a Master Portfolio. BlackRock Investments, LLC ("BRIL") is the placement agent for the Master Portfolios.

A non-accredited investor may not directly purchase an interest in a Master Portfolio, but instead may purchase shares in a feeder fund that invests directly in the Master Portfolio. Any accredited investors other than feeder funds that invest in the Master Portfolios will do so on the same terms and conditions as the feeder funds, although they may have different administrative and other expenses. Therefore, some indirect investors may have different returns than other indirect investors in the Master Portfolios.

The response to Item 13 has been omitted pursuant to General Instruction B, Paragraph 2(b), to Form N-1A under the 1940 Act.

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MASTER INVESTMENT PORTFOLIO

U.S. TOTAL BOND INDEX MASTER PORTFOLIO

S&P 500 INDEX MASTER PORTFOLIO

PART B - STATEMENT OF ADDITIONAL INFORMATION

April 30, 2026

Master Investment Portfolio ("MIP") is an open-end, series management investment company. MIP is a "series fund," which is a mutual fund company that has been divided into separate portfolios. This Part B is not a prospectus and should be read in connection with Part A, also dated April 30, 2026 of U.S. Total Bond Index Master Portfolio and S&P 500 Index Master Portfolio (each, a "Master Portfolio" and together, the "Master Portfolios"). All terms used in this Part B that are defined in Part A have the meanings assigned in Part A, unless otherwise defined in Part B. MIP incorporates by reference the information included on the cover page of the statement of additional information relating to iShares U.S. Aggregate Bond Index Fund and iShares S&P 500 Index Fund (each, a "BlackRock Funds III Feeder Fund" and together, the "BlackRock Funds III Feeder Funds"), as amended, revised or supplemented from time to time (the statement of additional information relating to iShares U.S. Aggregate Bond Index Fund, the "U.S. Aggregate SAI" and the statement of additional information relating to iShares S&P 500 Index Fund, the "S&P 500 SAI"). A copy of Part A of the Registration Statement with respect to the Master Portfolios may be obtained without charge by writing to Master Investment Portfolio, c/o BNY Mellon Investment Servicing (US) Inc., 118 Flanders Road Westborough, MA 01581, or by calling 1-800-441-7762. MIP's registration statement may be examined at the office of the Securities and Exchange Commission (the "SEC") in Washington, D.C.

NEITHER PART A NOR THIS PART B CONSTITUTES AN OFFER TO SELL, OR THE SOLICITATION OF AN OFFER TO BUY, ANY BENEFICIAL INTERESTS IN A MASTER PORTFOLIO.

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TABLE OF CONTENTS

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MIP is an open-end, series management investment company organized on October 20, 1993 as a statutory trust under the laws of the State of Delaware. MIP is a "series fund," which is a mutual fund company that has been divided into separate portfolios.

The following information supplements and should be read in conjunction with Item 9 of Part A.

INVESTMENTS AND RISKS. MIP incorporates by reference the information concerning each Master Portfolio's additional investment strategies, risks and restrictions from the following sections of the U.S. Aggregate SAI: "Part I: Investment Objective and Policies," "Part I: Investment Restrictions," "Part II: Investment Risks and Considerations" and "Appendix A - Description of Bond Ratings" and the following sections of the S&P 500 SAI: "Part I: Investment Objective and Policies," "Part I: Investment Restrictions," "Part II: Investment Risks and Considerations" and "Appendix A - Description of Bond Ratings".

PORTFOLIO HOLDINGS INFORMATION. MIP incorporates by reference the information concerning each Master Portfolio's policies and procedures with respect to the disclosure of portfolio holdings from the following section of the U.S. Aggregate SAI: "Part II: Selective Disclosure of Portfolio Holdings" and the following section of the S&P 500 SAI: "Part II: Selective Disclosure of Portfolio Holdings." Information on any significant variation or anticipated variation in each Master Portfolio's portfolio turnover rates, if any, is incorporated herein by reference to the following section of the U.S. Aggregate SAI: "Part I: Portfolio Transactions and Brokerage" and the following section of the S&P 500 SAI: "Part I: Portfolio Transactions and Brokerage."

The following information supplements and should be read in conjunction with Item 10 of Part A.

MIP incorporates by reference the information concerning the management of MIP and the Master Portfolios from the following section of the U.S. Aggregate SAI: "Part I: Information on Trustees and Officers" and the following section of the S&P 500 SAI: "Part I: Information on Trustees and Officers." The Board of Trustees has responsibility for the overall management and operations of the Master Portfolios. The Board of Trustees of MIP has the same chair and the same committee structure as the board of trustees of BlackRock Funds III (the "Trust").

COMPENSATION OF TRUSTEES. MIP incorporates by reference the information concerning the compensation of the Trustees of MIP from the following section of the U.S. Aggregate SAI: "Part I: Information on Trustees and Officers - Compensation of Trustees" and the following section of the S&P 500 SAI: "Part I: Information on Trustees and Officers - Compensation of Trustees."

CODES OF ETHICS. MIP has the same code of ethics as the Trust. MIP incorporates by reference the information concerning the code of ethics from the following section of the U.S. Aggregate SAI: "Part II: Management and Other Service Arrangements - Code of Ethics" and the following section of the S&P 500 SAI: "Part II: Management and Other Service Arrangements - Code of Ethics."

PROXY VOTING POLICIES. MIP incorporates by reference the information concerning its Proxy Voting Policies from the following sections of the U.S. Aggregate SAI: "Part II: Proxy Voting Policies and Procedures" and "Appendix B" and the following sections of the S&P 500 SAI: "Part II: Proxy Voting Policies and Procedures" and "Appendix B."

INTERESTHOLDER COMMUNICATION TO THE BOARD OF TRUSTEES. The Board of Trustees has established a process for interestholders to communicate with the Board of Trustees. Interestholders may contact

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the Board of Trustees by mail. Correspondence should be addressed to Master Investment Portfolio Board of Trustees, c/o BlackRock, Inc., 50 Hudson Yards, New York, New York 10001. Interestholder communication to the Board of Trustees should include the following information: (a) the name and address of the interestholder; (b) the percentage interest(s) owned by the interestholder; (c) the Master Portfolio(s) of which the interestholder owns interests; and (d) if these interests are owned indirectly through a broker, financial intermediary or other record owner, the name of the broker, financial intermediary or other record owner. All correspondence received as set forth above shall be reviewed by the Secretary of MIP and reported to the Board of Trustees.

As of April 6, 2026 the interestholders identified below were known by MIP to own 5% or more of the outstanding voting interests of a Master Portfolio in the indicated capacity. Approximate percentages are indicated in the table below.

For purposes of the 1940 Act, any person who owns directly or through one or more controlled companies more than 25% of the voting securities of a company is presumed to "control" such company. Accordingly, to the extent that a BlackRock Funds III Feeder Fund, or another interestholder is identified in the foregoing table as the beneficial holder of more than 25% of a Master Portfolio or as the holder of record of more than 25% of a Master Portfolio and has voting and/or investment powers, such interestholder may be presumed to control the Master Portfolio. Any feeder fund that is a majority interestholder in a Master Portfolio may be able to take actions with respect to MIP (e.g., approve an advisory agreement) without the approval of other investors in the applicable Master Portfolio.

As of April 2, 2026, no Trustee owned any beneficial interest in either Master Portfolio, and the Trustees and principal officers of MIP as a group owned an aggregate of less than 1% of the outstanding beneficial interests of each Master Portfolio. As of December 31, 2025, none of the Independent Trustees of MIP or their immediate family members owned beneficially or of record any securities of each Master Portfolio's investment adviser, principal underwriter, or any person directly or indirectly controlling, controlled by, or under common control with such entities.

The following information supplements and should be read in conjunction with Items 10 and 12 in Part A. Information relating to the investment management and other services provided to each Master Portfolio by BlackRock Fund Advisors ("BFA") is incorporated herein by reference from the section entitled "Management of the Fund" in the Prospectuses relating to iShares U.S. Aggregate Bond Index Fund (each a "U.S. Aggregate Prospectus") and the Prospectuses relating to iShares S&P 500 Index Fund (each an "S&P 500 Prospectus") and from the sub-section entitled "Management, Advisory and Other Service Arrangements " in Part I of the U.S. Aggregate SAI and the S&P 500 SAI and the section entitled "Management and Other Service Arrangements" in Part II of the U.S. Aggregate SAI and the S&P 500 SAI. The following list identifies the specific sections and sub-sections in each BlackRock Funds III Feeder Fund's SAI under which the information required by Item 19 of Form N-1A may be found. Each listed section is incorporated herein by reference.

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BlackRock Investments, LLC ("BRIL"), 50 Hudson Yards, New York, New York 10001, an affiliate of BFA, acts as placement agent for each Master Portfolio pursuant to a placement agent agreement (the "Placement Agent Agreement"). Under the Placement Agent Agreement, BRIL receives no compensation for acting as placement agent for the Master Portfolios.

MIP incorporates by reference the information concerning the portfolio managers for the Master Portfolios from the following section of the corresponding U.S. Aggregate SAI: "Part I: Management, Advisory and Other Service Arrangements - Information Regarding the Portfolio Managers" and the following section of the S&P 500 SAI: "Part I: Management, Advisory and Other Service Arrangements - Information Regarding the Portfolio Managers."

MIP incorporates by reference the information describing the Master Portfolios' policies governing portfolio securities transactions generally, portfolio turnover, brokerage commissions and frequent trading in portfolio

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securities from the following sections of the U.S. Aggregate SAI: "Part I: Portfolio Transactions and Brokerage" and "Part II: Portfolio Transactions and Brokerage" and the following sections of the S&P 500 SAI: "Part I: Portfolio Transactions and Brokerage" and "Part II: Portfolio Transactions and Brokerage."

Pursuant to MIP's Declaration of Trust, the Trustees are authorized to issue beneficial interests in the Master Portfolios. Interestholders in the Master Portfolios are entitled to participate pro rata in distributions and, generally, in allocations of income, gain, loss, deduction and credit of the Master Portfolios. Under certain circumstances, allocations of tax items to interestholders will not be made pro rata in accordance with their interests in the Master Portfolios in order to comply with tax rules and regulations applicable to such allocations. Upon liquidation or dissolution of a Master Portfolio, interestholders are entitled to share pro rata in the Master Portfolio's Net Assets available for distribution to its interestholders. Interests in the Master Portfolios have no preference, preemptive, conversion or similar rights and are fully paid and non-assessable, except as set forth below. Interests in the Master Portfolios may not be transferred. No certificates are issued. MIP may be terminated at any time by vote of interestholders holding at least a majority of the interests of each series entitled to vote or by the Trustees by written notice to the interestholders. Any series of interests may be terminated at any time by vote of interestholders holding at least a majority of the interests of such series entitled to vote or by the Trustees by written notice to the interestholders of such series.

Each interestholder is entitled to vote, with respect to matters affecting each of MIP's portfolios, in proportion to the amount of its investment in MIP. Interestholders in MIP do not have cumulative voting rights, and interestholders holding more than 50% of the aggregate beneficial interest in MIP may elect all of the Trustees if they choose to do so and in such event the other interestholders in MIP would not be able to elect any Trustee. MIP is not required to hold annual meetings of interestholders but MIP may hold special meetings of interestholders when in the judgment of the Trustees it is necessary or desirable to submit matters for interestholders' vote.

Rule 18f-2 under the 1940 Act provides that any matter required to be submitted under the provisions of the 1940 Act or applicable state law or otherwise to the holders of the outstanding voting interests of an investment company that issues two or more classes or series, such as MIP, will not be deemed to have been effectively acted upon unless approved by the holders of a majority of the outstanding interests of each class or series affected by such matter. Rule 18f-2 further provides that a class or series, such as each Master Portfolio, shall be deemed to be affected by a matter unless it is clear that the interests of each class or series in the matter are substantially identical or that the matter does not affect any interest of the class or series. However, Rule 18f-2 exempts the selection of independent accountants and the election of Trustees from the separate voting requirements of Rule 18f-2.

The following information supplements and should be read in conjunction with Item 11 in Part A.

PURCHASE OF INTERESTS. Beneficial interests in each Master Portfolio are issued solely in private placement transactions that do not involve any "public offering" within the meaning of Section 4(a)(2) of the 1933 Act. Investments in a Master Portfolio may only be made by investment companies or certain other entities that are "accredited investors" within the meaning of Regulation D under the 1933 Act. This registration statement does not constitute an offer to sell, or the solicitation of an offer to buy, any "security" within the meaning of the 1933 Act.

IN-KIND PURCHASES. Payment for interests of a Master Portfolio may, at the discretion of the investment adviser, be made in the form of securities that are permissible investments for the applicable Master Portfolio and must meet the investment objective, policies and limitations of the applicable Master Portfolio as described in

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Part A. In connection with an in-kind securities payment, a Master Portfolio may require, among other things, that the securities: (i) be valued on the day of purchase in accordance with the pricing methods used by the Master Portfolio; (ii) be accompanied by satisfactory assurance that the Master Portfolio will have good and marketable title to such securities received by it; (iii) not be subject to any restrictions upon resale by the Master Portfolio; (iv) be in proper form for transfer to the Master Portfolio; and (v) be accompanied by adequate information concerning the basis and other tax matters relating to the securities. All dividends, interest, subscription or other rights pertaining to such securities shall become the property of the Master Portfolio engaged in the in-kind purchase transaction and must be delivered to the Master Portfolio by the investor upon receipt from the issuer. Securities acquired through an in-kind purchase will be acquired for investment and not for immediate resale. Interests purchased in exchange for securities generally cannot be redeemed until the transfer has settled.

SUSPENSION OF REDEMPTIONS. The right of redemptions of interests in each Master Portfolio may be suspended or the date of redemptions payment postponed as provided in Item 11 in Part A.

VALUATION. MIP incorporates by reference information concerning the Master Portfolios' and MIP's pricing of interests from the following section of the U.S. Aggregate SAI: "Part II: Pricing of Shares - Determination of Net Asset Value" and the following section of the S&P 500 SAI: "Part II: Pricing of Shares - Determination of Net Asset Value."

DECLARATION OF TRUST PROVISIONS REGARDING REDEMPTIONS AT OPTION OF TRUST. Pursuant to the Declaration of Trust, MIP shall, subject to applicable law, have the right at its option and at any time to redeem interests of any interestholder at the net asset value thereof as determined in accordance with the Declaration of Trust (i) if at such time such interestholder owns fewer interests than, or interests having an aggregate net asset value of less than, an amount determined from time to time by the Trustees; or (ii) to the extent that such interestholder owns interests of a particular series or class equal to or in excess of a percentage of the outstanding interests of that series or class, or a percentage of the aggregate net asset value of that series or class, determined from time to time by the Trustees; or (iii) to the extent that such interestholder owns interests of MIP equal to or in excess of a percentage of the aggregate outstanding interests of MIP, or a percentage of the aggregate net asset value of MIP, as determined from time to time by the Trustees.

To the extent that a Master Portfolio has more than one Feeder Fund, it is treated as a non-publicly traded partnership under the Internal Revenue Code of 1986, as amended (the "Internal Revenue Code"). To the extent that a Master Portfolio has only one Feeder Fund (either (i) directly or (ii) indirectly through an entity that is disregarded for U.S. federal income tax purposes), the Master Portfolio's existence as an entity separate from that Feeder Fund is disregarded for U.S. federal income tax purposes.

Whether a Master Portfolio is a partnership or disregarded as a separate entity, it will not generally be subject to any U.S. federal income tax. Each of the Master Portfolios' Feeder Funds will take into account its allocable share of the applicable Master Portfolio's ordinary income, capital gains, losses, deductions and credits in determining its income tax liability and, if the Feeder Fund is intended to qualify as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code, the Feeder Fund's share of the Master Portfolio's income and assets.

It is intended that each Master Portfolio's assets, income and distributions will be managed in such a way that each Feeder Fund will be able to satisfy the requirements of Subchapter M of the Internal Revenue Code for qualification as a RIC, assuming that the Feeder Fund invests all of its investable assets in the applicable Master Portfolio and the Feeder Fund meets all other requirements for such qualification not within the control of the Master Portfolio.

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Certain transactions of each Master Portfolio are subject to special tax rules of the Internal Revenue Code that may, among other considerations, (a) affect the character of gains and losses realized, (b) disallow, suspend or otherwise limit the allowance of certain losses or deductions, and (c) accelerate the recognition of income without a corresponding receipt of cash (with which to make the necessary distributions to satisfy distribution requirements applicable to RICs). Operation of these rules could, therefore, affect the character, amount and timing of distributions to shareholders of a Feeder Fund.

Special tax rules also will require certain types of positions to be marked to market (i.e., treated as sold on the last day of the taxable year), which may result in the recognition of income without a corresponding receipt of cash.

If a Master Portfolio purchases shares of an investment company (or similar investment entity) organized under foreign law, each of its Feeder Funds, by virtue of owning such Master Portfolio's interests, will generally be treated as owning shares in a passive foreign investment company ("PFIC") for U.S. federal income tax purposes. A Feeder Fund may be subject to U.S. federal income tax (plus an interest charge) on certain distributions from such a PFIC and on gain from the disposition of the shares in such a PFIC (collectively referred to as "excess distributions"). A Feeder Fund, even if it is a RIC, cannot eliminate this tax by making distributions to its shareholders.

However, an election can be made to mark to market the interest in the PFIC, and thus to take into account the economic gains (and to a limited extent losses) in such investment as though the Feeder Fund's shares in the PFIC had been sold and repurchased on the last day of the Feeder Fund's taxable year. Such gain and loss are treated as ordinary income and loss. Alternatively, an election may be made to treat a PFIC as a "qualified electing fund" ("QEF"), in which case a Feeder Fund will be required to include in its gross income its share of the PFIC's ordinary earnings and net capital gain annually, regardless of whether it receives any distributions from the PFIC. With the mark-to-market or QEF election, a Feeder Fund could avoid imposition of the interest charge with respect to excess distributions from PFICs, but in any particular year might be required to recognize income in excess of the distributions received from PFICs.

If a Master Portfolio is treated as a partnership for U.S. federal income tax purposes, then, in the event of an audit, the U.S. federal income tax treatment of income and deductions of a Master Portfolio generally will be determined at the Master Portfolio level in a single proceeding (rather than by individual Internal Revenue Service audits of each Feeder Fund), which the Master Portfolio's partnership representative will control. Any adjustment that results in additional tax (including interest and penalties thereon) will be assessed and collected at the Master Portfolio level in the current taxable year, with each current Feeder Fund indirectly bearing such cost, unless the Master Portfolio elects to have the partnership adjustment taken into account by each Feeder Fund that was an investor in the Master Portfolio in the year to which the adjustments relates. If the election is made, each Feeder Fund will be required to take into account such adjustment and pay tax on such adjustment at the Feeder Fund level, unless, if the Feeder Fund is a RIC, it timely distributes the adjustment amount in the form of a "deficiency dividend" (within the meaning of section 860(f) of the Internal Revenue Code), in which case the Feeder Fund will only have to pay the interest charge imposed on RICs making a deficiency dividend. The legal and accounting costs incurred in connection with any regular audit of the Master Portfolio's tax returns will be borne by each Feeder Fund and, indirectly, by their shareholders.

Investors are advised to consult their own tax advisers on the tax consequences of an investment in a Feeder Fund and a Master Portfolio.

The exclusive placement agent for MIP is BRIL, which receives no compensation from the Master Portfolios for serving in this capacity. Registered broker-dealers and investment companies, insurance company separate accounts, common and commingled trust funds, group trusts and similar organizations and entities

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which constitute accredited investors, as defined in the regulations adopted under the 1933 Act, may continuously invest in a Master Portfolio. The foregoing information supplements and should be read in conjunction with Item 12 in Part A.

Not applicable.

The audited financial statements and financial highlights, including the schedules of investments, statement of assets and liabilities, statements of operations, statements of changes in net assets, and independent registered public accounting firm's reports for the BlackRock Funds III Feeder Funds and the Master Portfolios for the fiscal year ended December 31, 2025 are included in the Master Portfolios' Form N-CSR (SEC File No. 811-08162), as filed with the SEC on March 6, 2026 (the "2025 Annual Financial Statements"), and are incorporated in this Part B by reference. No other parts of the 2025 Annual Financial Statements are incorporated by reference herein. Additional copies of the Feeder Funds' and the Master Portfolios' Annual and Semi-Annual Reports to shareholders and the Feeder Funds' and the Master Portfolios' Annual and Semi-Annual Financial Statements are available upon request and without charge.

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PART C

OTHER INFORMATION

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The Registrant does not control and is not under common control with any other person.

Section 1 of Article IX of the Registrant's Second Amended and Restated Agreement and Declaration of Trust states:

"(a) Subject to the exceptions and limitations contained in paragraph (b) below: (i) every Person who is, or has been, a Trustee or officer of the Trust (including any individual who serves at its request as director, officer, partner, trustee or the like of another organization in which it has any interest as a shareholder, creditor or otherwise) (hereinafter referred to as a "Covered Person") shall be indemnified by the Trust, or by one or more Series thereof if the claim arises from his or her conduct with respect to only such Series (unless the Series was terminated prior to any such liability or claim being known to the Trustees, in which case such obligations, to the extent not satisfied out of the assets of a Series, the obligation shall be an obligation of the Trust), to the fullest extent permitted by law against liability and against all expenses reasonably incurred or paid by such Covered Person in connection with any claim, action, suit, or proceeding in which such Covered Person becomes involved as a party or otherwise or is threatened to be involved as a party or otherwise by virtue of being or having been a Trustee or officer and against amounts paid or incurred by such Covered Person in the settlement thereof; and (ii) the words "claim," "action," "suit," or "proceeding" shall apply to all claims, actions, suits, or proceedings (civil, criminal, regulatory or other, including investigations and appeals), actual or threatened, while in office or thereafter, and the words "liability" and "expenses" shall include, without limitation, attorney's fees, costs, judgments, amounts paid in settlement, fines, penalties, and other liabilities.

(b) No indemnification shall be provided hereunder to a Covered Person: (i) who shall have been adjudicated by a court or body before which the proceeding was brought (A) to be liable to the Trust or its Interestholders by reason of willful misfeasance, bad faith, gross negligence, or reckless disregard of the duties involved in the conduct of the Covered Person's office or (B) not to have acted in good faith in the reasonable belief that his or her action was in the best interest of the Trust; or (ii) in the event of a settlement or other disposition not involving a final adjudication as provided in paragraph (b)(i) resulting in a payment by a Covered Person, unless there has been a determination that such Trustee or officer did not engage in willful misfeasance, bad faith, gross negligence, or reckless disregard of the duties involved in the conduct of his or her office: (A) by the court or other body approving the settlement or other disposition; (B) by at least a majority of those Trustees who neither are Interested Persons of the Trust nor are parties to the matter based upon a review of readily-available facts (as opposed to a full trial-type inquiry); or (C) by written opinion of independent legal counsel based upon a review of readily-available facts (as opposed to a full trial-type inquiry).

(c) The rights of indemnification herein provided may be insured against by policies maintained by the Trust, shall be severable, shall not be exclusive of or affect any other rights to which any Covered Person may now or hereafter be entitled, shall continue as to a person who has ceased to be a Covered Person and shall inure to the benefit of the heirs, executors, and administrators of such a Covered Person. Nothing contained herein shall affect any rights to indemnification to which Trust personnel, other than Covered Persons, and other persons may be entitled by contract or otherwise under law.

(d) To the maximum extent permitted by applicable law, expenses in connection with the preparation and presentation of a defense to any claim, action, suit, or proceeding of the character described in paragraph (a) of this Article IX, Section 1 may be paid by the Trust or Series from time to time prior to final disposition thereof upon receipt of an undertaking by or on behalf of such Covered Person that such amount will be repaid by such Covered Person to the Trust or Series if it ultimately is determined that he or she is not entitled to indemnification under this Article IX, Section 1; provided, however, that either (i) such Covered Person shall have provided a surety bond or some other appropriate security for such undertaking;

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(ii) the Trust or Series thereof is insured against losses arising out of any such advance payments, or (iii) either a majority of the Trustees who are neither Interested Persons of the Trust nor parties to the matter, or independent legal counsel in a written opinion, shall have determined, based upon a review of readily-available facts (as opposed to a trial-type inquiry or full investigation), that there is a reason to believe that such Covered Person will be entitled to indemnification under this Article IX, Section 1. In connection with any determination pursuant to clause (iii) of the preceding sentence, any Covered Person who is a Trustee and is not an Interested Person of the Trust and any Covered Person who has been a Trustee and at such time was not an Interested Person of the Trust shall be entitled to a rebuttable presumption that he or she has not engaged in willful misfeasance, bad faith, gross negligence, or reckless disregard of the duties involved in the conduct of his or her office.

(e) Any repeal or modification of this Article IX, Section 1, or adoption or modification of any other provision of this Declaration or the By-Laws inconsistent with this Section, shall be prospective only, to the extent that such repeal, or modification adoption would, if applied retrospectively, adversely affect any limitation on the liability of any Covered Person or indemnification available to any Covered Person with respect to any act or omission which occurred prior to such repeal, adoption or modification."

(a) BlackRock Fund Advisors (previously defined as "BFA") is an indirect majority-owned subsidiary of BlackRock, Inc. located at 400 Howard Street, San Francisco, California 94105. BFA's business is that of a registered investment adviser to certain open-end, management investment companies and various other institutional investors. The information required by this Item 31 about officers and directors of BFA, together with information as to any other business, profession, vocation or employment of a substantial nature engaged in by such officers and directors during the past two years, is incorporated by reference to Schedules A and D of Form ADV, filed by BFA pursuant to the Investment Advisers Act of 1940, as amended (File No. 801-22609).

(b) BlackRock Advisors, LLC (previously defined as "BAL") is an indirect majority-owned subsidiary of BlackRock, Inc. located at 100 Bellevue Parkway, Wilmington, Delaware 19809. BAL was organized in 1994 for the purpose of providing advisory services to investment companies. The information required by this Item 31 about officers and directors of BAL, together with information as to any other business, profession, vocation or employment of a substantial nature engaged in by such officers and directors during the past two years, is incorporated by reference to Schedules A and D of Form ADV, filed by BAL pursuant to the Investment Advisers Act of 1940 (SEC File No. 801-47710).

(c) BlackRock International Limited (previously defined as "BIL") is located at Dundas House, 20 Brandon Street, Edinburgh, EH3 5PP Scotland. The information required by this Item 31 about officers and directors of BIL, together with information as to any other business, profession, vocation or employment of a substantial nature engaged in by such officers and directors during the past two years, is incorporated by reference to Schedules A and D of Form ADV, filed by BIL pursuant to the Investment Advisers Act of 1940 (SEC File No. 801-51087).

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(a) BlackRock Investments, LLC (previously defined as "BRIL"), the placement agent of the Master Portfolios, acts as the principal underwriter or placement agent, as applicable, for each of the following open-end registered investment companies including the Registrant:

BRIL also acts as the distributor or placement agent for the following closed-end registered investment companies:

BRIL provides numerous financial services to BlackRock-advised funds and is the distributor of BlackRock's open-end funds. These services include coordinating and executing Authorized Participation Agreements, preparing, reviewing and providing advice with respect to all sales literature and responding to Financial Industry Regulatory Authority comments on marketing materials.

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(b) Set forth below is information concerning each director and officer of BRIL. The principal business address for each such person is 50 Hudson Yards, New York, New York 10001.

(c) Not applicable.

Omitted pursuant to Instruction 3 of Item 33 of Form N-1A.

Other than as set forth under the captions "Item 10. Management, Organization and Capital Structure" in Part A of this Registration Statement, and "Item 17. Management of the Trust" and "Item 19. Investment Advisory and Other Services" in Part B of this Registration Statement, the Registrant is not a party to any management-related service contract.

Not applicable.

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SIGNATURES

Pursuant to the requirements of the Investment Company Act of 1940, the Registrant certifies that it has duly caused this Amendment to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and the State of New York, on April 30, 2026.

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EXHIBIT INDEX