ASHINGTON, March 19 — Two years ago, long before most Americans gave much thought to bioterrorism, a small group of scientists began a series of experiments they hoped would yield a treatment for smallpox.

It seemed a highly theoretical task, because smallpox was effectively wiped out in 1980 and the possibility that it would recur as a biological weapon appeared slim.

On Wednesday, nearly six months after the anthrax attacks made the prospect of smallpox seem frighteningly real, the researchers will report the early fruits of their work: a medicine that stops replication of the smallpox virus in the laboratory dish and that prevents symptoms in mice infected with a cousin of smallpox.

Because the drug has yet to be tested for safety in people, it may be years before it will be considered for government approval.

And one leading expert said the experiments did not address the critical question of how the medicine might work in someone already exhibiting the characteristic smallpox lesions.

Still, the studies are generating excitement among scientists, who see a possible therapy for one of the world's most dreaded scourges.

"Although it's too early to say that this is, quote, a breakthrough, there are some very encouraging components about this," said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. "We are pushing very vigorously to get this into a trial in humans, to see if it would work."

The drug, developed by scientists at the University of California at San Diego and the Army's bioterror defense laboratory at Fort Detrick, Md., is a potent derivative of an existing antiviral medicine, cidofovir [sigh-DOFF-oh-veer]. Marketed by Gilead Sciences of Foster City, Calif., the medicine is ordinarily used to treat a complication of AIDS, and was recently approved by the Food and Drug Administration as an experimental treatment for smallpox in the event of a bioterror attack.

But cidofovir must be administered intravenously, making it impractical in a fast-moving outbreak. The derivative, which is made by adding a lipid, or fat, to cidofovir, is 100 times as powerful as the original drug and can be given in pill form, said Dr. Karl Y. Hostetler, a professor of medicine at the University of California at San Diego who developed it.

"If it tracks all the way through, it could be a very useful addition to our armamentarium," Dr. Hostetler said. "People could self-administer it instead of having to go to places and stand in line."

The research has not yet been published in an academic journal, so it has not been subjected to the independent evaluation known as peer review. Dr. Hostetler and his chief collaborator, Dr. John Huggins of Fort Detrick, plan to announce their results on Wednesday at a medical conference in Prague.

Smallpox is highly contagious and kills 30 percent of its victims. Although the disease was officially declared to be eradicated in 1980, the United States and Russia still keep stocks of the smallpox virus, and defense experts believe certain other nations may be developing it for use as a weapon.

In this country, routine vaccination against smallpox ended in 1972. But last fall, after the Sept. 11 terrorist attacks, the federal government began to stockpile the vaccine.

The vaccine can prevent smallpox up to four days after exposure to the virus. But it does not work for everyone. Once people develop symptoms, it is too late. And the vaccine is risky for people with weakened immune systems.

So Dr. Huggins said he envisioned the cidofovir derivative as "a second line of defense for those people who are vaccinated too late or cannot be vaccinated."

Because the drug has also shown promise against herpes and cytomegalovirus, a complication of AIDS, Dr. Huggins and Dr. Hostetler said they hoped the drug would be marketed for those conditions and stockpiled in the event of a smallpox attack. The University of California at San Diego, which holds the patent on the derivative, said today that it was negotiating with a pharmaceutical company interested in licensing the drug.

Dr. D. A. Henderson, who led the worldwide effort to eradicate smallpox, said he was skeptical of any stockpiling plan.

Dr. Henderson, who directs the federal office charged with preparing the public health response to bioterrorism, opposes efforts to stockpile the original cidofovir because animal studies show the drug is effective only immediately after exposure to smallpox, but not once symptoms appear. If the derivative works the same way, Dr. Henderson said, he saw little advantage over the vaccine.

"I know I look like some kind of troglodyte, as though I am not interested in anything novel or new," he said. "But you can see why I am a little lukewarm about this."

In any event, Dr. Henderson and Dr. Hostetler said the conversation about stockpiling was premature. The next step is to test the derivative in monkeys, and then people. For Dr. Hostetler, at least, the terrorist attacks of Sept. 11 and the anthrax attacks that followed have given the work a profound sense of urgency.

"It was all very theoretical," he said, "until those planes crashed into the buildings."