Chiron Will Not Supply BEGRIVAC(TM) Influenza Virus Vaccine to Non-U.S. Markets for the 2005-2006 Influenza Season; Company is W

                    No Change to Financial Guidance

   Chiron Corporation (NASDAQ:CHIR) today announced that further
testing has confirmed the company will not be able to supply any
BEGRIVAC(TM) influenza virus vaccine doses to non-U.S. markets during
the 2005-2006 influenza season. There are no other changes from the
company's previous announcement on July 15, 2005, on the same subject.

   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 BEGRIVAC(TM) influenza virus vaccine
and other influenza virus vaccines that Chiron expects to deliver to
non-U.S. markets, and assumptions 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, the outcome of clinical trials, 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 the BEGRIVAC or
FLUVIRIN vaccines or Chiron's manufacturing generally will not arise
in the future, or that Chiron will be able to cover vaccine
shortfalls, or will successfully address matters raised in a warning
letter from the U.S. Food and Drug Administration (FDA) with respect
to its FLUVIRIN vaccine manufacturing facility or resume sale of
FLUVIRIN vaccine for the 2005-2006 influenza season or BEGRIVAC
vaccine for the 2006-2007 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 the recent
BEGRIVAC and FLUVIRIN 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: BEGRIVAC and FLUVIRIN are trademarks of Chiron Corporation.