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

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.