ASHINGTON, Oct. 17 — Federal health officials are negotiating with four drug companies to buy 300 million doses of smallpox vaccine — enough for every American — and are gingerly discussing the possibility that ordinary Americans might someday once again be vaccinated against the disease.

While there are no immediate plans for vaccination, Tommy G. Thompson, the secretary of health and human services, said today that he was asking lawmakers for $509 million so that the government could stockpile enough vaccine to protect everyone in the nation against the potentially lethal smallpox virus.

"I can report to you that it looks very promising that we will have the 300 million doses by sometime next year," Mr. Thompson told reporters this evening. He said that he had met with representatives of two drug companies today, and that he would meet with two others on Thursday.

The decision to pursue more stocks of smallpox vaccine came as fears of attack with another biological agent, anthrax, spread across the country. Mr. Thompson did not say whether the recent spate of anthrax- laced letters, including one mailed to Tom Daschle, the Senate majority leader, influenced his decision.

Nor did he say whether the government was aware of any specific threat involving smallpox. Asked what prompted the policy shift, Mr. Thompson simply replied, "We thought we should go and see if we could get some other companies interested in the 300 million doses."

The health secretary and President Bush have been discussing bioterrorism preparedness since the Sept. 11 attacks on the World Trade Center and Pentagon, said Kevin Keane, a spokesman for Mr. Thompson. Sometime within the past week, Mr. Keane said, the White House gave Mr. Thompson the go-ahead to ask Congress to pay for the 300 million doses, so the country could be prepared for "the worst-case scenario."

Unlike anthrax, which is not transmitted from person to person, smallpox is highly contagious, and Mr. Thompson is clearly aware that it poses a potential menace. After the Sept. 11 attacks, he named Dr. Donald A. Henderson, who led the global effort that resulted in the eradication of smallpox in 1979, to lead a new advisory council on bioterrorism.

In June, a war game with the code name Dark Winter showed what chaos could erupt from a bioterrorist attack involving smallpox. The exercise, at Andrews Air Force Base, outside Washington, began with a report of a single case of smallpox in Oklahoma City. By the time it was over, the imaginary epidemic had spread to 25 states and killed several million people. As it unfolded, growing grimmer and grimmer, the government quickly ran out of vaccine.

Smallpox vaccine can be used not only to prevent infection with the smallpox virus but also to treat people exposed to the virus, and thus contain an epidemic.

But the United States abandoned smallpox vaccinations in 1972, because the disease had been virtually wiped out here. The vaccine itself carries serious health risks: It produced adverse reactions in roughly 1 in 13,000 vaccinated people, ranging from severe rashes to brain inflammation, which killed about one person in one million. So experts said that the risk of vaccination was greater than the risk of getting the disease.

On Oct. 3, Mr. Thompson said his agency had negotiated with Acambis, a British company that makes the vaccine, to deliver 40 million doses by next summer, and not in 2004 or 2005 as originally planned. Today, he said that the Acambis contract has been expanded to 54 million doses, and that his department was conducting additional negotiations with Acambis, as well as negotiating with Merck & Company, one of the major vaccine makers in the United States. Mr. Thompson said his department was also negotiating with Baxter International, as well as another company he did not name.

Experts say that the chances that terrorists could lay hands on the smallpox virus — which officially exists now only in government laboratories in the United States and Russia — are remote.

"It's impossible to quantify but I would assess it to be quite low," said Jonathan B. Tucker, an expert in bioterrorism and the author of "Scourge: The Once and Future Threat of Smallpox," (Atlantic Monthly Press, 2001).

But smallpox, which kills about one of three people infected with it, is a particularly worrisome threat because it is easily transmittable. Even those Americans who have been vaccinated are at risk, because the vaccine's protection is believed to last only 15 or 20 years.

And unlike anthrax, which would require that spores be prepared according to precise specifications to infect large numbers of people, a smallpox epidemic could begin with a single infected person — a "smallpox martyr," in the terminology of bioterrorism experts — simply walking through a crowd.

The two official laboratory repositories for smallpox, one at the Centers for Disease Control and Prevention in Atlanta and the other at a Russian government facility in western Siberia, are monitored by the World Health Organization, and are generally thought to be secure.

But, Dr. Tucker said, there are "suspicions that there are undeclared stocks of smallpox virus in Russia," particularly at a top-secret virology laboratory under control of the Russian Ministry of Defense. A federal intelligence report completed in 1998 concluded that clandestine stocks of smallpox virus probably existed in Russia, as well as in Iraq and North Korea.

The growing fear of bioterrorism has renewed the question of whether Americans should be vaccinated. Today, a higher proportion of the population than in the past may run the risk of being harmed by the vaccine. It could cause serious illness in people whose immune systems are suppressed, including organ transplant recipients and people with AIDS, a disease that was not even known the last time Americans were vaccinated for smallpox.

So a decision to vaccinate Americans would not be made lightly, and Mr. Thompson emphasized today that there are no plans to do so.

But he did suggest that the government would revisit the question, a startling comment in and of itself. "Sometime in the future there may be a discussion that may lead to voluntary vaccination for the smallpox bug," he said.

Should that happen, it would constitute a huge shift in public policy. Mohammad N. Akhter, executive director of the American Public Health Association, has been publicly urging the administration to reopen the question of smallpox vaccination.

"My worry is that there will be a case in the U.S., we will rush to contain it, we will immunize some people but the level of public concern and the demand will be such that we will not be in a position to make a thoughtful decision," Dr. Akhter said today. He said the scientific community needed to "rethink the immunization priority for our people against smallpox."

The renewal of the immunization debate, said Dr. Irwin Redlener, president of a children's hospital at Montefiore Medical Center in the Bronx, "makes us really all pause in terms of what has happened to our country."

Dr. Redlener added, "If we have to go back to vaccinating people for smallpox, we are really about to turn back the hands of time."