UPDATES OF ATrynŽ CLINICAL DEVELOPMENT AND MARKET ESTIMATES

FRAMINGHAM, MA - May 24, 2007 -- GTC Biotherapeutics, Inc. ("GTC", Nasdaq: GTCB) held its annual meeting of shareholders yesterday. Directors Robert W. Baldridge, James A. Geraghty, and Michael J. Landine were elected to three year terms on the board of directors. In addition, the proposed amendment and restatement of GTC's 2002 Equity Incentive Plan was approved.

During a presentation to shareholders following the business portion of the meeting, GTC updated its progress in the ATrynŽ program, including updating the estimated market in the acquired deficiency indication of disseminated intravascular coagulation, or DIC, in severe sepsis.

"ATrynŽ has the potential to become an effective treatment to reduce the current mortality rate, which can be up to 50% in patients with disseminated intravascular coagulation associated with severe sepsis," commented Suzanne Groet, Vice President of Therapeutic Development for GTC. "The needs of the approximately 500,000 patients spread equally between the U.S. and Europe who develop DIC associated with severe sepsis each year are not well met by current treatments. The opportunity for ATrynŽ is to ultimately demonstrate a significant reduction in mortality and displace the costs associated with current treatment options."

ATrynŽ is GTC's recombinant form of human antithrombin. Antithrombin, produced in the liver and circulated in blood plasma, has both anticoagulant and anti-inflammatory properties. The recombinant form is able to be produced using GTC's transgenic production platform, enabling large volume supply. ATrynŽ has been approved for use in Europe for the prophylactic treatment of deep vein thrombosis in hereditary antithrombin deficient patients undergoing surgical procedures. LEO Pharma A/S, GTC's partner in the development and commercialization of ATrynŽ in Europe, Canada, and the Middle East, is initiating a Phase II dose ranging study in the DIC indication to establish an optimum dose to be used in a subsequent Phase III study. Recruitment into the Phase II study is anticipated to begin this quarter and is planned to take about 12 months.

In evaluating the clinical and commercial opportunities in DIC in the U.S., GTC has determined that existing treatment options for severe sepsis are priced at a benchmark of approximately $10,000 per treatment, which if applied to DIC indicates a total potential U.S. market of $2 to 3 billion per year. GTC believes that ATrynŽ has the potential to achieve in excess of $1 billion of this potential DIC market in the U.S. alone, if successfully developed and approved for DIC associated with severe sepsis.

GTC is also nearing completion of the clinical studies necessary to support the filing of a Biologics License Application for ATrynŽ in the HD indication planned for the first quarter of 2008. These studies include an evaluation of the incidence of deep vein thrombosis and thromboembolisms in 17 additional hereditary antithrombin deficient patients undergoing surgical and pregnancy procedures that will be treated with ATrynŽ and a non-inferiority comparison to a total of 35 historical records of patients undergoing similar procedures that have been treated with plasma-derived antithrombin products. GTC plans on releasing top line data from these studies in the fourth quarter of 2007. GTC is in discussions for potential commercial partnering in the US market for ATrynŽ.

The full presentation from the annual shareholder meeting can be accessed at the GTC web site, www.gtc-bio.com.

About GTC Biotherapeutics
GTC Biotherapeutics develops, produces, and commercializes therapeutic proteins through transgenic animal technology. In August 2006, ATrynŽ, GTC's recombinant form of human antithrombin, was approved by the European Commission for use in patients with hereditary antithrombin deficiency undergoing surgical procedures. This was the first approval anywhere in the world of a therapeutic protein produced from a transgenic animal. We have developed goats that have the human antithrombin gene linked to a milk-protein promoting gene so that they express this protein in their milk. This transgenic approach provides the opportunity to produce recombinant forms of proteins, such as antithrombin, that are difficult to express in economically viable quantities in conventional production systems. In addition, GTC has established a strategic collaboration with LFB Biotechnologies of France to jointly develop recombinant forms of human plasma proteins and monoclonal antibodies. The first program of the collaboration will be to develop recombinant human factor VIIa as a potential treatment for hemophilia in patients with antibodies to other coagulation factors.

In 2006, GTC was granted a patent in the United States through 2021 for the production of any therapeutic protein in the milk of any transgenic mammal. GTC's transgenic production platform is particularly well suited to enabling cost effective development of proteins that are difficult to express in traditional recombinant production systems as well as those that are required in large volumes. Additional information is available on the GTC web site, 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 prospects for clinical development of the DIC indication as well as the ultimate potential market size in the U.S. in that 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 filed with the Securities and Exchange Commission, including the uncertainties associated with conducting clinical studies, and the risks and uncertainties associated with dependence upon the actions of collaboration partners and regulatory agencies. 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.

CONTACT:
GTC Biotherapeutics, Inc.
Thomas E. Newberry
Vice President, Corporate Communications
(508) 370-5374