Summary Prospectus by Investment Company (Form 497K)

SUMMARY PROSPECTUS

December 29, 2025

MFS^® U.S. Government Money Market Fund

Before you invest, you may want to review the fund's prospectus, which contains more information about the fund and its risks. You can find the fund's prospectus and other information about the fund, including the fund's reports to shareholders and statement of additional information, online at funds.mfs.com. You can also get this information at no cost by calling 1-800-225-2606 or by sending an e-mail request to [email protected]. The fund's prospectus and statement of additional information, both dated December 29, 2025, as may be amended or supplemented from time to time, are incorporated by reference into this summary prospectus.

Summary of Key Information

Investment Objective

The fund's investment objective is to seek current income consistent with preservation of capital and liquidity.

Fees and Expenses

This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below.

¹ Massachusetts Financial Services Company (MFS) has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least December 31, 2026. MFS has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.45% of the fund's average daily net assets annually. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least December 31, 2026.

MMM-SUM-122925

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Example

This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds.

The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund's operating expenses remain the same.

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Principal Investment Strategies

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests at least 99.5% of the fund's total assets in cash, U.S. Government money market instruments, and/or repurchase agreements collateralized by cash or U.S. Government securities. MFS normally invests at least 80% of the fund's net assets in U.S. Government money market instruments and repurchase agreements collateralized by U.S. Government securities. U.S. Government money market instruments are short-term instruments issued or guaranteed as to the payment of principal and interest by the U.S. Treasury, by an agency or instrumentality of the U.S. Government, or by a U.S. Government-sponsored entity that pay a fixed, variable, or floating interest rate. U.S. Government securities include mortgage-backed securities and other types of asset-backed securities issued or guaranteed by the U.S. Treasury, by an agency or instrumentality of the U.S. Government, or by a U.S. Government-sponsored entity.

In buying and selling investments for the fund, MFS follows U.S. Securities and Exchange Commission (SEC) rules for money market funds regarding credit quality, diversification, liquidity, and maturity. MFS stresses maintaining a stable $1.00 per share price, liquidity, and income.

For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes.

Principal Risks

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not a bank account and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. The fund's sponsor is not required to reimburse the fund for losses, and you should not expect that the sponsor will provide financial support to the fund at any time, including during periods of market stress.

The principal risks of investing in the fund are:

Interest Rate Risk: In general, the price of a money market instrument falls when interest rates rise and rises when interest rates fall. A major or unexpected increase in interest rates could cause the fund's share price to decrease to below $1.00 per share. In addition, a low or negative interest rate environment may prevent the fund from providing a positive yield and could impair the fund's ability to maintain a stable $1.00 per share.

Credit and Market Risk: The value of a money market instrument depends on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment. The value of a money market instrument can also decline in response to changes in, or perceptions of, the financial condition of the issuer or borrower, changes in, or perceptions of, specific market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions that affect a particular type of instrument, issuer, or borrower, and changes in, or perceptions of, general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions.

Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results.

Redemption Risk: Large or frequent redemptions could cause the fund's share price to decrease to below $1.00 per share.

Focus Risk: If MFS invests a significant percentage of the fund's assets in a single issuer or small number of issuers, the fund's performance could be more volatile than the performance of more diversified funds.

Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty's or third party's ability or willingness to perform in accordance with the terms of the transaction.

Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs.

Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would.

Performance Information

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund's performance over time.

The fund's past performance does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606.

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Bar Chart.

The total return for the nine-month period ended September 30, 2025, was 2.94%. During the period(s) shown in the bar chart, the highest quarterly return was 1.25% (for the calendar quarter ended December 31, 2023) and the lowest quarterly return was 0.00% (for the calendar quarter ended March 31, 2022).

Performance Table.

Investment Adviser

MFS serves as the investment adviser for the fund.

Purchase and Sale of Fund Shares

You may purchase and redeem shares of the fund each day the New York Stock Exchange (the NYSE) is open for trading. You may purchase or redeem shares either by having your financial intermediary process your purchase or redemption, or through MFS Service Center, Inc. (MFSC) by overnight mail (MFSC, Suite 219341, 801 Pennsylvania Avenue, Kansas City, MO 64105-1307), by mail ([Fund Name], P.O. Box 219341, Kansas City, MO 64121-9341), by telephone (1-800-225-2606), or via the Internet at mfs.com (MFS Access).

The fund's initial and subsequent investment minimums generally are as follows:

Effective at the close of business on May 29, 2020 (the Closing Date), the fund is closed to all purchases except: 1) purchases through the reinvestment of distributions in the fund; 2) purchases as a result of exchanges from another MFS fund; 3) purchases as a result of shareholders exercising their one-time Reinstatement Privilege to reinvest in shares of the fund within 90 days of a redemption of fund shares; 4) purchases in connection with a shareholder's transfer of some or all of the shares in the shareholder's account to another account; 5) purchases by retirement plans other than traditional IRAs, SIMPLE IRAs, Roth IRAs, SEP plans, and SARSEP plans if the fund was offered or officially selected as an investment option by such plan (or any predecessor plan or other plan sponsored by the same employer) as of the Closing Date; 6) purchases by qualified tuition programs established under Section 529 of the Internal Revenue Code of 1986, as amended, if the fund was offered as an investment option by such program on the Closing Date; and 7) investors approved by the fund, the fund's investment adviser, and the fund's distributor.

The fund reserves the right to modify or limit the above exceptions or re-open the fund at any time without prior notice.

Financial intermediaries are responsible for enforcing these restrictions with respect to their investors. MFS' ability to monitor financial intermediaries' enforcement of these restrictions is limited by operational systems and the cooperation of financial intermediaries. In addition, with respect to omnibus accounts, MFS' ability to monitor is also limited by a lack of information with respect to the underlying shareholder accounts.

Taxes

If your shares are held in a taxable account, the fund's distributions will be taxed to you as ordinary income and/or capital gains. The fund expects that substantially all its distributions will consist of investment income taxable as ordinary income. However, if your shares are held in a tax-advantaged account, you will generally be taxed only upon withdrawals from the account unless such an account allows for tax-free withdrawals.

Payments to Broker/Dealers and Other Financial Intermediaries

If you purchase shares of the fund through a broker/dealer or other financial intermediary (such as a bank), the fund, MFS, and/or MFS' affiliates may pay the financial intermediary for the sale of shares of the fund and/or the servicing of shareholder accounts. These payments may create a conflict of interest by influencing your broker/dealer or other financial intermediary and your salesperson to recommend the fund over another investment. Ask your financial intermediary or visit your financial intermediary's website for more information.

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