The Food and Drug Administration gave VaxGen Inc. permission to conduct human trials of its experimental anthrax vaccine, news that sent shares of the Brisbane biotech company soaring 93 percent in after-hours trading.

The FDA action keeps VaxGen in the running for lucrative government contracts to produce a stockpile of as much as 25 million vaccine doses in case of another anthrax outbreak. If VaxGen wins the full stockpile contract, it could bring in revenue estimated at between $100 million to $250 million.

The anthrax vaccine work has already provided a shot in the arm for VaxGen, whose stock price plummeted in February on disappointing results for its core product, an AIDS vaccine. VaxGen and a partner are sharing a $16.2 million government grant to develop an anthrax vaccine based on scientific discoveries at the U.S. Army Medical Research Institute of Infectious Diseases.

The VaxGen product is vying for the stockpile contract with a contender that is also receiving U.S. government support to develop its anthrax vaccine candidate -- the Avecia Group PLC of Britain.

Both competitors must follow standard FDA procedures to win commercial marketing approval for their vaccines. VaxGen scored the FDA's go-ahead for the first step in that process. It can now test the vaccine on 100 people to see whether it is safe.

The VaxGen and Avecia vaccines are both based on a recombinant form of a protein called protective antigen, which induces antibodies that neutralize anthrax toxins. The clinical trials approved by the FDA will measure formation of those antibodies, but no subjects will be exposed to the anthrax germ to test the effectiveness of the vaccine.

The government began its drive for development of a new anthrax vaccine after letters laced with the deadly microbe were sent to media organizations and members of Congress in late 2001. Anthrax spores released from the letters killed five people.

In regular trading, VaxGen shares closed up 20 cents at $3.34. But on news of the FDA action, the shares gained 92.81 percent to $6.44 in after-hours trading.