GTC Biotherapeutics Receives $1 Million Milestone Payment from LEO Pharma.

FRAMINGHAM, Mass. -- GTC Biotherapeutics, Inc. ("GTC", Nasdaq: GTCB) announced today that it has received a $1 million payment from LEO Pharma A/S for achieving the milestone of a positive opinion on the market authorization application for ATryn(R) from the Committee for Medicinal Products for Human Use (CHMP) of the European Medicines Agency (EMEA). The CHMP has recommended that ATryn(R), GTC's recombinant form of human antithrombin, be granted market authorization for the prophylaxis of venous thromboembolism in surgery of patients with congenital antithrombin deficiency. Final market authorization by the European Commission, which is expected in about three months, will trigger an additional $2 million milestone payment from LEO Pharma.

In November 2005, GTC and LEO Pharma entered into a collaboration to develop and market ATryn(R) in Europe, Canada, and the Middle East. In addition to marketing and sales of ATryn(R) for any approved indication in these territories, LEO is responsible for additional clinical development of ATryn(R) in Europe. GTC is responsible for ATryn(R) production and will receive a transfer payment from LEO for all product used in additional clinical studies in the LEO territories, in addition to transfer payments and royalties on any future commercial sales in the LEO territories.

The EMEA's review process was the first regulatory examination of a transgenically produced therapeutic protein anywhere in the world and is the only available antithrombin product that is not derived from the human blood supply. Upon approval, ATryn(R) will also be the first antithrombin product approved for use in all 25 countries of the European Union.

"This milestone payment is an important marker of achieving the positive opinion," stated Geoffrey F. Cox, Ph.D., GTC's Chairman of the Board and Chief Executive Officer. "We look forward to working with LEO on developing the commercial launch of ATryn(R) as well as continuing development in larger market indications."

LEO has begun development of ATryn(R) in Europe as a potential treatment for disseminated intravascular coagulation, or DIC, associated with severe sepsis. DIC occurs in an estimated 220,000 severe sepsis cases in the European Union each year, of which approximately 50% are fatal, representing a major unmet medical need of significant interest in critical care. LEO has requested scientific advice from the EMEA as the basis for clinical studies in DIC.

GTC anticipates using the results from both the completed study of hereditary deficient patients reviewed by the CHMP and GTC's ongoing pivotal phase III study to prepare a Biologics License Application for the United States Food and Drug Administration, which is planned for mid-2007.

Antithrombin is a protein in human plasma that has anticoagulant and anti-inflammatory properties.

About GTC Biotherapeutics, Inc.

GTC Biotherapeutics is a leader in the development, production, and commercialization of therapeutic proteins through transgenic animal technology. In addition to the ATryn(R) program, GTC has in development a recombinant human alpha-1 antitrypsin, a recombinant human albumin, a CD137 antibody to stimulate the immune system as a potential treatment for solid tumors, and a malaria vaccine. In its external programs, GTC's technology is used to develop transgenic production of its partners' proprietary products, including both large-volume protein therapeutics as well as products that are difficult to produce in significant quantities from conventional recombinant production systems. One of GTC's external programs is in clinical trials with a transgenically produced product. Additional information is available on the GTC web site, http://www.gtc-bio.com.

This press release contains forward-looking statements as defined in the Private Securities Litigation Reform Act of 1995, including without limitation statements regarding its expectations for the European Commission approval of ATryn(R) for the hereditary deficiency indication, the preparation of a BLA for filing with the FDA, and potential future clinical studies of ATryn(R) for the DIC indication. Such forward-looking statements are subject to a number of risks, uncertainties and other factors that could cause actual results to differ materially from future results expressed or implied by such statements. Factors that may cause such differences include, but are not limited to, the risks and uncertainties discussed in GTC's most recent Annual Report on Form 10-K and its other periodic reports as filed with the Securities and Exchange Commission, including the uncertainties associated with clinical trials and regulatory review of therapeutic products. GTC cautions investors not to place undue reliance on the forward-looking statements contained in this release. These statements speak only as of the date of this document, and GTC undertakes no obligation to update or revise the statements, except as may be required by law.