Human Genome Sciences Inc. plans to announce a deal today under which the federal government might buy as many as 100,000 doses of an experimental drug to treat anthrax, potentially giving the nation a new defense against one of the most feared agents of biological terrorism.

The initial government order will be for a small sample of the drug, known as ABthrax, with the option to buy much more later. The large order is by no means assured, and other companies developing similar drugs contend that their products are superior and say they still have a chance to beat Human Genome Sciences.

The contract, which the Rockville company plans to announce before stock markets open this morning, does suggest that the government is moving forward to develop a new way to save lives if the deadly anthrax attacks of late 2001 are ever repeated.

Company executives said the Department of Health and Human Services will pay the Rockville company $1.8 million for a third of an ounce of the drug, a sample large enough to analyze against competing products in development. Jerry Parrot, the company's spokesman, said the purchase price covers the product and testing support for the government.

"The price for the product is not inexpensive, particularly in these small quantities," he said.

The contract would allow the government, if it is satisfied with those tests, to buy 10,000 to 100,000 doses under the $5.6 billion Project BioShield program.

A large purchase could total more than $200 million, providing a significant revenue surge for the 13-year-old firm, which has yet to bring a product to market. Like other biotechnology firms, it raced to produce a treatment for anthrax after the lethal toxin was used in attacks four years ago, killing five people, sickening dozens of others and disrupting Capitol Hill, the federal government, the U.S. Postal Service and many companies that were forced to change their mailroom operations.

ABthrax is an artificial antibody, a type of drug that could be used to boost the immune system of a person exposed, or likely to be exposed, to anthrax spores. That drug, or a similar product from another company, might become one of a panoply of treatments designed to counter anthrax attacks.

The National Institutes of Health has awarded grants to support development of similar products at several small companies. The government may be setting itself up to pick the best, or perhaps the cheapest, of several similar anthrax treatments.

Since 2001, the government has been buying various drugs and vaccines to augment a national stockpile meant to safeguard against attacks with germs, chemicals or radiation. The administration has been under heavy pressure from Capitol Hill to show progress on anthrax and other biological agents, but federal scientists have resisted committing to drugs without test data to know how well they would work. Frustrated biotech companies have complained that the testing and procurement process is too slow.

"The government has a difficult road to walk," said Lance K. Gordon, chief executive of VaxGen Inc., which was awarded $1 billion in contracts to develop a vaccine that could be used to immunize people against anthrax before or just after an attack. "They need to encourage innovation. They need to stimulate the development of new technologies. But they also have an absolute need, mandated by Congress, to get something now."

The 25 million doses of VaxGen's experimental vaccine might be used to immunize some people -- medical personnel, for example -- well before an attack. It might also be used just after an attack to protect people who might be exposed. They would probably have to take antibiotics, which can prevent anthrax infection, but the vaccine -- which is a next-generation version of the Bioport anthrax vaccine already in use in the military -- would provide extra protection.

Noreen A. Hynes, a senior official at the Health and Human Services Department, said the vaccine strategy alone isn't enough. The government wants to stockpile drugs that could stop the damage from anthrax even after a person is infected, possibly saving lives.

That's where ABthrax or similar drugs might come in. Testing by Human Genome Sciences shows that animals given ABthrax immediately before anthrax exposure are able to fend off infection. Many animals given the drug up to 36 hours later also survived. ABthrax has already been shown safe in 105 adult volunteers, the company said.

Perhaps the leading competitor to Human Genome Sciences for an anthrax contract is Elusys Therapeutics Inc. of Pine Brook, N.J. It has been working on a similar product since 2000, before Human Genome got involved, and has received nearly $20 million in government grants for the work.

Elizabeth G. Posillico, president and chief executive of Elusys, said in an interview last week that her company's anthrax antibody, called Anthim, is more potent than the one developed by Human Genome Sciences and would probably cost the government less per dose.

David C. Stump, executive vice president of drug development for Human Genome Sciences, said "it's entirely way too soon to know which drug will be more potent or cost-effective."

With support from the National Institutes of Health, Elusys has tested its drug in rabbits, in which it proved effective, and is on the verge of launching a study in humans, Posillico said.

"I think we're still very much in the running" for an anthrax contract, Posillico said, adding that she thought the government was "trying to be prudent" by funding studies of multiple anthrax products so it can pick the best.

Another company angling for an anthrax contract is PharmAthene Inc. of Annapolis, which, with Medarex Inc. of Princeton, N.J., hopes to develop an artificial antibody. The firms recently received a go-ahead from the Food and Drug Administration to begin human testing, also funded by the NIH.