FDA On-Site Inspection of Chiron's FLUVIRIN(R) Influenza Virus Vaccine Facility Underway; Full cGMP Inspection Process Expected to Last Weeks.

EMERYVILLE, Calif. -- Chiron Corporation (NASDAQ:CHIR) today announced that the U.S. Food and Drug Administration (FDA) is currently conducting a full cGMP (current Good Manufacturing Practices) inspection of Chiron's Liverpool facility, which manufactures FLUVIRIN(R) influenza virus vaccine. Following the on-site inspection, Chiron expects to respond to any observations noted by the FDA. Chiron expects that the FDA will then assess Chiron's response in conjunction with the agency's findings. Chiron will communicate the outcome of the full inspection once this process has concluded and the FDA has issued formal results, which may occur several weeks after the on-site inspection has ended.

Chiron currently expects to produce 18 to 26 million doses of FLUVIRIN vaccine for the 2005-2006 influenza season, with the vast majority of the supply going to the U.S. market. This dose range assumes that Chiron is successful in its remediation efforts; that the company encounters no further adverse manufacturing or regulatory developments; and that the major factors that determine production, including volumes, yields and timing, develop as anticipated. In addition to the facility inspection, Chiron will need to receive supplemental approvals for changes in its product from the Medicines and Healthcare products Regulatory Agency (MHRA) and FDA because of variations to its manufacturing process. The Center for Biologics Evaluation and Research (CBER), a division of the FDA, validates and releases FLUVIRIN vaccine for the U.S. market.

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, please visit www.chiron.com.

This news release contains forward-looking statements, including statements regarding supply of FLUVIRIN(R) influenza virus vaccine that Chiron expects to deliver to the U.S. market in future influenza seasons, sales and earnings expectations, and improvements to manufacturing facilities, that involve risks and uncertainties and are subject to change. A discussion of the company's operations and financial condition, including factors that may affect its business and future prospects that could cause actual results and developments to differ materially from those expressed or implied by forward-looking statements, is contained in documents the company has filed with the SEC, including the Form 10-K for the year ended December 31, 2004, and the Form 10-Q for the quarter ended March 31, 2005, and will be contained in all subsequent periodic filings made with the SEC. These documents identify important factors that could cause the company's actual performance to differ from current expectations, including, among others, additional adverse developments resulting from the suspension of Chiron's UK license to manufacture FLUVIRIN vaccine from October 5, 2004, through March 2, 2005, the announcement of such suspension and the litigation and investigations relating to those matters, regulatory review and approvals, manufacturing capabilities, intellectual property protections and defenses, litigation, stock-price and interest-rate volatility, marketing effectiveness, and the severity of the 2005-2006 influenza season. In particular, there can be no assurance that additional issues with respect to FLUVIRIN vaccine or Chiron's manufacturing generally will not arise in the future or that Chiron will successfully address matters raised in a warning letter from the FDA with respect to its FLUVIRIN vaccine manufacturing facility or resume sale of FLUVIRIN vaccine for the 2005-2006 influenza season. In addition, the company may face additional competition in the influenza market in the future and challenges in distribution arrangements as a result of recent influenza vaccine developments. In addition, the company may engage in business opportunities, the successful completion of which is subject to certain risks, including approval by Novartis AG, shareholder and regulatory approvals, and the integration of operations.

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

NOTE: FLUVIRIN is a trademark of Chiron Corporation.