Antigenics Appoints Peter Thornton as Chief Financial Officer

NEW YORK — May 25, 2004 — Antigenics Inc. (NASDAQ: AGEN) today announced that Peter Thornton will join the company as chief financial officer and senior vice president. Jeff Clark, who has been CFO for the past year, has resigned, with plans to return to his home state of Texas.

“We are very pleased to have an executive of Peter’s qualifications and experience join Antigenics as CFO,” said Garo H. Armen, PhD, chairman and CEO of Antigenics. “I have had the opportunity to see Peter’s work at Elan; his strong finance and operations experience will be very beneficial to our company as we get ready for the commercial launch of our products. Jeff has done an outstanding job as CFO and we are sad to see him go. We thank him for his contributions, and wish him and his family well.”

Mr. Thornton brings to Antigenics 17 years of financial and operating experience in the United States and Europe. He joins the company having held senior management positions in accounting, finance and operations at Elan Corporation, plc, for the past decade. Most recently, he served as senior vice president of business operations at Elan Enterprises, where he managed the restructuring and divestiture of businesses in the United States and Europe. Prior to Elan, Mr. Thornton worked for KPMG in Dublin and Paris. Mr. Thornton is a fellow of the Institute of Chartered Accountants in Ireland and received his bachelor of commerce degree from University College Cork, Ireland.

About Antigenics

Antigenics is working to develop personalized immunotherapeutics and revolutionary treatments for cancers, infectious diseases and autoimmune disorders. The company’s lead product in development is Oncophage^® (HSPPC-96), a late-stage, personalized cancer vaccine being evaluated in several indications, including renal cell carcinoma and metastatic melanoma. Antigenics’ portfolio of investigational products also includes AG-858 (HSPPC-70), a personalized cancer vaccine in Phase 2 development; two Phase 2 liposomal cancer treatments, Aroplatin™ and ATRA-IV; and AG-702/AG-707, a Phase 1 genital herpes program.

This press release contains forward-looking statements, including statements regarding the benefits of Mr. Thornton joining Antigenics and the expected commercial launch of products. These statements are subject to risks and uncertainties that could cause actual results to differ materially from those projected in these forward-looking statements. Risks and uncertainties relating to Mr. Thornton’s future contributions to Antigenics include the transition of responsibilities from Mr. Clark to Mr. Thornton, and the time period during which Mr. Thornton remains at Antigenics. Risks and uncertainties relating to commercialization of Oncophage and our other product candidates include, among others, the ability to satisfy the FDA and other regulatory authorities regarding product characterization, the results of clinical trials, the ability to successfully prepare Oncophage, and the factors described in the company’s periodic filings with the Securities and Exchange Commission. Please see the disclosure under the heading “Factors That May Impact Future Results” in the Management’s Discussion and Analysis of Financial Condition and Results of Operations section of the Antigenics Quarterly Report on Form 10-Q for the fiscal quarter ending March 31, 2004, for a more complete discussion of these and other risk factors. Antigenics cautions investors not to place considerable reliance on the forward-looking statements contained in this press release. These statements speak only as of the date of this document, and Antigenics undertakes no obligation to update or revise the statements. All forward-looking statements are expressly qualified in their entirety by this cautionary statement. Antigenics’ business is subject to substantial risks and uncertainties, including those identified above. When evaluating Antigenics’ business and securities, investors should give careful consideration to these risks and uncertainties.