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): March 5, 2007

ImmunoGen, Inc.

(Exact name of registrant as specified in its charter)

128 Sidney Street, Cambridge, MA 02139

(Address of principal executive offices)   (Zip Code)

Registrant’s telephone number, including area code: (617) 995-2500

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 5.02. — DEPARTURE OF DIRECTORS OR PRINCIPAL OFFICERS; ELECTION OF DIRECTORS; APPOINTMENT OF PRINCIPAL OFFICERS.

(a) Not applicable.

(b) On March 6, 2007, ImmunoGen, Inc. (Nasdaq: IMGN) announced the March 5, 2007 departure of Walter Blättler, Ph.D., formerly the Executive Vice President, Science and Technology, from the Company in accordance with Section 2, paragraph (b)(ii)(C) of his employment agreement, which was filed with ImmunoGen’s Quarterly Report on Form 10-Q, for the period ended December 31, 2006.

A copy of the press release regarding the departure is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

(c), (d), and (e) Not applicable.

ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS

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.

EXHIBIT INDEX

128 Sidney Street, Cambridge, MA 02139-4239                                      TEL: (617) 995-2500    FAX: (617) 995-2510

Contacts:

For Immediate Release

ImmunoGen, Inc. Announces Departure of Walter Blättler

CAMBRIDGE, MA, March 6, 2007 — ImmunoGen, Inc. (Nasdaq: IMGN) today announced the departure of Walter Blättler, Ph.D., formerly the Executive Vice President, Science and Technology, of the Company.

“Walter has made a tremendous contribution to ImmunoGen over his many years with the Company. He has been the intellectual force behind our immunoconjugate technology and is responsible for much of the Company’s success in reaching our current stage of development. We wish him the very best in his future endeavors,” commented Mitchel Sayare, Ph.D., Chairman and CEO.

The Company has initiated a search for a pharmaceutical/biotech executive with extensive experience in research management and product development in addition to strong scientific expertise. In the interim, Dr. Blättler’s day-to-day responsibilities will be handled by John Lambert, Ph.D., Senior Vice President, Pharmaceutical Development, Pauline Jen Ryan, Senior Vice President, Corporate Development and Operations, and Dr. Sayare.

About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer biopharmaceuticals. The Company’s proprietary Tumor-Activated Prodrug (TAP) technology uses tumor-targeting antibodies to deliver a potent cell-killing agent specifically to cancer cells. Two TAP compounds wholly owned by ImmunoGen are in clinical testing — huN901-DM1 and huC242-DM4. Companies licensing the right to develop anticancer compounds to specific targets using ImmunoGen’s TAP technology include Biogen Idec, Biotest AG, Centocor (Johnson & Johnson), Genentech, and sanofi-aventis. Three anticancer compounds are in clinical testing through ImmunoGen’s collaborations with other companies — AVE9633 and AVE1642, in development by sanofi-aventis, and trastuzumab-MCC-DM1, in development by Genentech.

# # #