FIRST AMENDMENT TO
STOCK PURCHASE AGREEMENT
BY AND AMONG
BIOTECHNOLOGY VENTURE PARTNERS, L.P.,
BIOTECHNOLOGY VALUE FUND, L.P.,
BIOTECHNOLOGY VALUE FUND, LTD.
INVESTMENT 10, L.L.C.
This FIRST AMENDMENT TO THE STOCK PURCHASE AGREEMENT (the
"Agreement") made on the 10th day of December, 1997 by and among ImmunoGen, Inc., a Massachusetts
corporation (the "Company"), Biotechnology Venture Partners, L.P. ("BVP"), a Delaware limited partnership,
Biotechnology Value Fund, L.P. ("BVF 1"), a Delaware limited partnership, Biotechnology Value Fund, Ltd.
("BVF 2"), a Cayman Island corporation and Investment 10, L.L.C. ("I10"), an Illinois limited liability company
(collectively BVP, BVF 1, BVF 2 and I10 are referred to as the "Investor") is made as of the 18th day of March,
1998 (this "Amendment").
WHEREAS, the Company desires to issue and sell to Investor and Investor desires to acquire shares (the
"Preferred Shares") of the Company's Series E convertible preferred stock, par value $.01 per share, convertible
into shares of the Company's Common Stock, par value $.01 per share ("Common Stock"), and having the
designations, powers, preferences, and other terms set forth on EXHIBIT A to the Agreement;
WHEREAS, the Company desires to issue and sell to Investor and Investor desires to acquire common stock
purchase warrants substantially in the form of EXHIBIT B to the Agreement (each, a "Warrant" and collectively,
the "Warrants") to purchase shares of the Company's common stock, par value $.01 per share (the "Warrant
WHEREAS, the Company and the Investor desire to amend the terms concerning the closings for the above-
NOW, THEREFORE, in consideration of the premises and other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, the parties agree as follows:
1. SECTION 1.4. Section 1.4 of the Agreement is hereby deleted in its entirety and shall be amended to read as