THIS AGREEMENT, is made as of December 9, 1997 among ImmunoGen, Inc., a Massachusetts Corporation
(the "Company"), Biotechnology Venture Partners, L.P. ("BVP"), a _____________ limited partnership,
Biotechnology Value Fund, L.P. ("BVF 1"), a Delaware limited partnership, Biotechnology Value Fund, Ltd.
("BVF 2"), a ________________________ and Investment 10 L.L.C. ("I10"), a _____________ limited
liability company (collectively, BVP, BVF 1, BVF 2, and (I10 are referred to as the "Investor").
The parties to this Agreement are parties to a Stock Purchase Agreement of even date herewith (the "Purchase
Agreement"). In order to induce the Investor to enter into the Purchase Agreement, the Company has agreed to
provide the registration rights set forth in this Agreement. The execution and delivery of this Agreement is a
condition to the Closing under the Purchase Agreement. Unless otherwise provided in this Agreement, capitalized
terms used herein shall have the meanings set forth in paragraph 8 hereof.
The parties hereto agree as follows:
1. DEMAND REGISTRATIONS.
(a) REQUESTS FOR REGISTRATION. For a period of two (2) years following the first issuance of the
Registrable Securities, either the Investor or any of its affiliates holding Registrable Securities may request
registration under the Securities Act of all or any portion of their Registrable Securities, provided, however, that
only one such request may be made with respect to registration of the Initial Registrable Securities and only one
such request may be made with respect to the registration of the Additional Registrable Securities. Each such
registration shall be on Form S-1 or any similar long-form registration for which the Company may qualify;
provided that the Company may, at its election, register all or any portion of such Registrable Securities on Form
S-2 or S-3 or any similar short-form registration if available. All registrations requested pursuant to this paragraph