XOMA Ltd. (Nasdaq:XOMA) and Chiron Corporation (Nasdaq:CHIR) today announced that several abstracts on the novel oncology compound CHIR-12.12, an antagonist antibody targeting CD40 currently in preclinical development, will be presented at the 46th American Society of Hematology (ASH) Annual Meeting and Exposition in San Diego, California, December 4 -7, 2004. In March 2004, XOMA and Chiron announced a collaborative agreement for the development and commercialization of antibody products for the treatment of cancer, including CHIR-12.12.

The following CHIR-12.12 abstract will be presented during the Novel Immune-Based Therapies for CLL poster session on Sunday, December 5, 2004:

-- "In Vitro Activity of a Novel Fully Human Anti-CD40 Antibody CHIR-12.12 in Chronic Lymphocytic Leukemia: Blockade of CD40 Activation and Induction of ADCC" (Poster Session No. 717-II)

The following CHIR-12.12 abstract will be presented during the Myeloma Therapy - Non-Transplant III poster session on Sunday, December 5, 2004:

-- "A Fully Human Antagonist Anti-CD40 Antibody Triggers Significant Antitumor Activity Against Human Multiple Myeloma" (Poster Session No. 627-II)

Three additional CHIR-12.12 abstracts will be presented during the Lymphoma Therapy - New Biologic Agents poster session on Monday, December 6, 2004:

-- "A Fully Human Anti-CD40 Antagonistic Antibody, CHIR-12.12, Inhibits the Proliferation of Human B Cell Non-Hodgkins Lymphoma" (Poster Session No. 549-III)

-- "Antagonist Anti-CD40 Monoclonal Antibody, CHIR-12.12, Inhibits Growth of a Rituximab-Resistant NHL Xenograft Model and Achieves Synergistic Activity When Combined with Ineffective Rituximab" (Poster Session No. 551-III)

-- "Safety Evaluation of an Fully Human Antagonist Anti-CD40 Antibody, CHIR-12.12, in a Dose Range-Finding Study in Cynomolgus Monkeys" (Poster Session No. 552-II)

Two other CHIR-12.12 abstracts will appear only in the publication book of abstracts available at the conference. They include:

-- "Antagonist Anti-CD40 Antibody CHIR-12.12 Causes Tumor Regression and Prolongs Survival in Multiple Myeloma Xenograft Models"

-- "Immunological Profile and Safety Evaluation in a 23-Week Single Dose Study in Cynomolgus Monkey with CHIR-12.12, a Fully Human Antagonist Anti-CD40 Antibody"

All CHIR-12.12 abstracts being presented at ASH can be found at the American Society of Hematology website at www.hematology.org.

About CHIR-12.12

In vitro, CHIR-12.12 has demonstrated dual mechanisms of B cell tumor killing: antibody-dependent cellular cytotoxicity (ADCC) of CD40-expressing tumors by immune effector cells and inhibition of CD40-ligand mediated growth and survival. Phase 1 studies in B-cell malignancies are planned for 2005.

About XOMA

XOMA is a biopharmaceutical company focused on the development and commercialization of antibody and other protein-based biopharmaceuticals for disease targets that include cancer, immunological and inflammatory disorders, and infectious diseases. XOMA's proprietary and collaborative product development programs include: RAPTIVA(R) for moderate to severe plaque psoriasis (marketed) and other indications in collaboration with Genentech, Inc.; MLN 2222, a recombinant protein for reducing the incidence of post-operative events in coronary artery bypass graft surgery patients with Millennium Pharmaceuticals, Inc. (Phase I); CHIR-12.12, an anti-CD40 antibody for treating B-cell tumors and additional product candidates in connection with an antibody oncology collaboration with Chiron Corporation (preclinical); a TPO mimetic antibody to treat chemotherapy-induced thrombocytopenia in collaboration with Alexion Pharmaceuticals, Inc. (preclinical); and anti-gastrin antibody product candidates in conjunction with the antibody collaboration for the treatment of gastrointestinal cancers with Aphton Corporation (preclinical). For more information about XOMA's product pipeline and antibody product development capabilities and technologies, please visit XOMA's website at http://www.xoma.com/.

About Chiron

Chiron delivers innovative and valuable products to protect human health by advancing pioneering science across the landscape of biotechnology. The company works to deliver on the limitless promise of science and make a positive difference in people's lives. For more information about Chiron, please visit www.chiron.com.

XOMA Forward-Looking Statement

Certain statements contained herein related to licensing arrangements or product development or that otherwise relate to future periods are forward-looking statements within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. These statements are based on assumptions that may not prove accurate. Actual results could differ materially from those anticipated due to certain risks inherent in the biotechnology industry and for companies engaged in the development of new products in a regulated market. These and other risks, including those related to the results of discovery research and preclinical testing; the timing or results of pending and future clinical trials (including the design and progress of clinical trials; safety and efficacy of the products being tested; action, inaction or delay by the FDA, European or other regulators or their advisory bodies; and analysis or interpretation by, or submission to, these entities or others of scientific data); changes in the status of the existing licensing arrangements or collaborative relationships; the ability of licensees, collaborators and other partners to finance their operations and otherwise meet their obligations; market demand for products; scale up and marketing capabilities; competition; international operations; share price volatility; XOMA's financing needs and opportunities; uncertainties regarding the status of biotechnology patents; uncertainties as to the cost of protecting intellectual property; and risks associated with XOMA's status as a Bermuda company, are described in more detail in XOMA's most recent annual report on Form 10-K and in other SEC filings.

Chiron Forward-Looking Statement

This news release contains forward-looking statements, including statements regarding product development initiatives, new product indications and planned clinical studies, that involve risks and uncertainties and are subject to change. A full discussion of Chiron's operations and financial condition, including factors that may affect its business and future prospects generally, is contained in documents Chiron has filed with the SEC, including the form 10-Q for the quarter ended September 30, 2004, and the form 10-K for the year ended December 31, 2003, and will be contained in all subsequent periodic filings made with the SEC. These documents identify important factors that could cause Chiron's actual performance to differ materially from current expectations, including those expressed or implied by the forward-looking statements contained in this press release. In particular, there can be no assurance that the companies will successfully develop CHIR-12.12. Other factors include the outcome of clinical trials, regulatory review and approvals, manufacturing capabilities, intellectual property protections and defenses, stock-price and interest-rate volatility, and marketing effectiveness. There can be no assurance that Chiron's out-licensing activities will generate significant revenue, nor that its in-licensing activities will fully protect it from claims of infringement by third parties. In addition, Chiron may engage in business opportunities, the successful completion of which are subject to certain risks, including shareholder and regulatory approvals and the integration of operations.

Chiron does not undertake an obligation to update the forward-looking information it is giving today.