ImmunoGen, Inc. (Nasdaq: IMGN) announced today that it intends to offer for sale its common stock in an underwritten public offering. The offering is subject to market and other conditions, and there can be no assurance as to whether or when the offering may be completed, or as to the actual size or terms of the offering. Morgan Stanley & Co. LLC and Jefferies & Company, Inc. are acting as joint book-running managers for the proposed offering. The securities described above are being offered by ImmunoGen pursuant to an automatically effective shelf registration statement that was previously filed with the Securities and Exchange Commission (SEC). This press release does not constitute an offer to sell or a solicitation of an offer to buy the securities in this offering, nor shall there be any sale of these securities in any state or other jurisdiction in which such offer, solicitation or sale would be unlawful prior to the registration or qualification under the securities laws of any such state or other jurisdiction. A preliminary prospectus supplement related to the offering will be filed with the SEC and will be available on the SEC’s website located at www.sec.gov. Copies of the preliminary prospectus supplement and the accompanying prospectus relating to this offering may be obtained, when available, from Morgan Stanley & Co. LLC, Attn: Prospectus Department, 180 Varick Street, 2nd Floor, New York, NY 10014, by telephone at 866-718-1649, or by email at firstname.lastname@example.org and from Jefferies & Company, Inc., Equity Syndicate Prospectus Department, 520 Madison Avenue, 12th Floor, New York, NY 10022, by telephone at 877-547-6340, or by email at Prospectus_Department@Jefferies.com. About ImmunoGen, Inc. ImmunoGen, Inc. develops targeted anticancer therapeutics using the Company’s expertise in tumor biology, monoclonal antibodies, potent cancer-cell killing agents and engineered linkers. The Company’s Targeted Antibody Payload (TAP) technology uses monoclonal antibodies to deliver one of ImmunoGen’s proprietary cancer-cell killing agents specifically to tumor cells. There are now ten TAP compounds in clinical development, of which three are wholly owned by ImmunoGen. ImmunoGen’s collaborative partners include Amgen, Bayer HealthCare, Biotest, Lilly, Novartis, Roche and Sanofi. The most advanced compound using ImmunoGen’s TAP technology, trastuzumab emtansine (T-DM1), is in Phase III testing through the Company’s collaboration with Genentech, a member of the Roche Group.