UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): May 20, 2011

ImmunoGen, Inc.

(Exact name of registrant as specified in its charter)

830 Winter Street, Waltham, MA 02451

(Address of principal executive offices)   (Zip Code)

Registrant’s telephone number, including area code: (781) 895-0600

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

o            Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o            Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o            Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o            Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 1.01.  ENTRY INTO A MATERIAL DEFINITIVE AGREEMENT

On May 20, 2011, ImmunoGen, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with Jefferies & Company, Inc., as representative of the several underwriters (the “Underwriters”) named in Schedule 1 of the Underwriting Agreement, related to a public offering of 7,000,000 shares of the Company’s common stock, par value $0.01 per share (the “Common Stock”), at a price of $12.00 per share less the underwriting commission (the “Offering”). Under the terms of the Underwriting Agreement, the Company has granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 1,050,000 shares of Common Stock to cover over-allotments, if any, at the same price.  The Offering is expected to close on May 25, 2011, subject to the satisfaction of customary closing conditions.  The net proceeds to the Company are expected to be approximately $78.9 million after deducting estimated expenses associated with the Offering.

The Offering is being made pursuant to a prospectus supplement dated May 20, 2011 and an accompanying prospectus dated May 19, 2011, pursuant to the Company’s automatically effective shelf registration statement on Form S-3 (File No. 333-174335), which was filed with the Securities and Exchange Commission (the “Commission”) on May 19, 2011.

The Underwriting Agreement contains customary representations, warranties, and agreements by the Company, and customary conditions to closing, indemnification obligations of the Company and the Underwriters, including for liabilities under the Securities Act of 1933, as amended, other obligations of the parties, and termination provisions.

A copy of the Underwriting Agreement is filed as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated herein by reference.  A copy of the opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. relating to the legality of the issuance and sale of the shares in the Offering is attached as Exhibit 5.1 to this Current Report on Form 8-K.  The foregoing description of the Offering by the Company and the documentation related thereto does not purport to be complete and is qualified in its entirety by reference to such Exhibits.

ITEM 9.01.  FINANCIAL STATEMENTS AND EXHIBITS

(d) The following exhibits are being filed herewith:

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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EXHIBIT INDEX

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