ZymoGenetics Begins Phase 3 Clinical Study with Recombinant Human Thrombin; Conference Call Today at 11:00 A.M. Eastern.

SEATTLE -- ZymoGenetics, Inc. (Nasdaq:ZGEN) today announced the start of a pivotal Phase 3 clinical study of recombinant human Thrombin (rhThrombin) in patients undergoing surgery. The study will compare the efficacy of rhThrombin to bovine-derived thrombin and is designed to support broad product labeling for the use of rhThrombin as an aid to controlling blood loss during surgery.

The randomized, double-blinded Phase 3 clinical trial with rhThrombin is being conducted at approximately 35 sites and will evaluate the same four types of surgery examined in Phase 2 studies: spinal, liver resection, peripheral artery bypass and arteriovenous graft construction. The primary objective of the trial is to assess efficacy by comparing the effectiveness of rhThrombin as a surgical hemostat to the currently approved bovine-derived thrombin, as measured by the incidence of hemostasis within 10 minutes. The secondary objective is to evaluate safety, including testing for the formation of antibodies against rhThrombin or bovine-derived thrombin. The study will enroll a minimum of 400 patients, approximately half of whom will receive rhThrombin.

"The thrombin market is attractive, and the need exists for a safer method of hemostasis. We believe that our rhThrombin may address that need," said Bruce L. A. Carter, Ph.D., President and Chief Executive Officer of ZymoGenetics. "We've begun our Phase 3 clinical trial and are on track to file a BLA by the end of 2006."

William C. Chapman, M.D., F.A.C.S., Washington University School of Medicine, and Phase 2 clinical investigator, commented, "Patients need safer treatments for surgical bleeding. Because recombinant products can be manufactured to high levels of purity and don't contain animal blood products, I believe rhThrombin could replace plasma-derived bovine thrombin."

In clinical trials to date, rhThrombin has been well tolerated with a favorable immunogenicity profile. Results from Phase 2 studies with rhThrombin were presented at several medical meetings in 2005, and the combined results of all four Phase 2 studies were presented at the International Society on Thrombosis and Haemostasis (ISTH) meeting in August 2005.

ZymoGenetics will conduct a conference call/webcast in conjunction with today's release at 11:00 a.m. ET on Thursday, October 27, 2005. Members of the news media, investors and the general public are invited to access a live broadcast of the conference call via the ZymoGenetics website at http://www.zymogenetics.com/ir/events.php or through www.vcall.com. The event will be archived. The telephone replay will be available until October 30, 2005, and the webcast will be available until November 28, 2005.

Phone numbers and access codes for the conference call are as

follows:

Live Call -- Domestic: 800-370-0740; International: 973-409-9255

Playback (Digital Replay) Dial-In -- Toll-Free DIRECT --

877-519-4471

Playback (Digital Replay) Dial-In -- Local DIRECT -- 973-341-3080

Conference ID # (required for playback): 6336549

About rhThrombin

ZymoGenetics is developing rhThrombin for the control of bleeding associated with surgical procedures. Thrombin is used in over 700,000 surgeries per year in the United States. Currently, only thrombin derived from bovine blood is available in the U.S. as a stand-alone thrombin product. Bovine-derived thrombin has been associated with the development of antibodies that may cross-react with human blood proteins. In some cases, these antibodies lead to serious bleeding complications. Recombinant proteins such as rhThrombin can be manufactured to higher levels of purity and consistency than proteins purified from human or animal blood. Furthermore, the production of recombinant proteins is not dependent on the availability of blood from human or animal donors and can be scaled-up to meet market demands. For these reasons, ZymoGenetics believes that rhThrombin represents a potential preferred alternative to the bovine-derived product.

About ZymoGenetics

ZymoGenetics is a biopharmaceutical company focused on the discovery, development and commercialization of therapeutic proteins for the prevention or treatment of human diseases. The Company is developing a diverse pipeline of potential proprietary product candidates that are moving into and through clinical development. These span a wide array of clinical opportunities that include bleeding, autoimmune diseases and cancer. ZymoGenetics intends to commercialize these product candidates through internal development, collaborations with partners, and out-licensing of patents from its extensive patent portfolio. For further information, visit www.zymogenetics.com.

This press release contains "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995. These forward-looking statements are based on the current intent and expectations of the management of ZymoGenetics. These statements are not guarantees of future performance and involve risks and uncertainties that are difficult to predict. ZymoGenetics' actual results and the timing and outcome of events may differ materially from those expressed in or implied by the forward-looking statements because of risks associated with our unproven discovery strategy, preclinical and clinical development, regulatory oversight, intellectual property claims and litigation and other risks detailed in the company's public filings with the Securities and Exchange Commission, including the company's Annual Report on Form 10-K for the year ended December 31, 2004. Except as required by law, ZymoGenetics undertakes no obligation to update any forward-looking or other statements in this press release, whether as a result of new information, future events or otherwise.