ASHINGTON, Oct. 18 — Dr. Marjorie Barnett, a pediatrician in Silver Spring, Md., is accustomed to talking to parents about immunization. These days, amid growing fear of bioterrorism, those conversations center not on hepatitis or polio, but on a disease that was wiped out decades ago: smallpox.

"I'm getting 5 or 10 calls a day from parents who want the vaccine," said Dr. Barnett, who believes that smallpox vaccinations, suspended in this country in 1972, should resume. "I have to tell them it's just not available. What I am telling them now is, `Call your Congress people, call the White House, call everybody you know and tell them to look into this.' "

One day after the federal government announced that it was laying plans to stockpile enough smallpox vaccine, 300 million doses, to protect every American if necessary, questions are quickly emerging about how that stockpile would be used.

The vaccine carries risks. A recent analysis of six million people who received their first smallpox vaccinations in 1968 found that six had died as a result, mostly of encephalitis, an inflammation of the brain. Other adverse effects can be far more common. Even so, some patients argue that they should be allowed to decide for themselves.

But public-health experts maintain that a mass vaccination program is unwise. Smallpox vaccine, they say, should be used only in the event of a smallpox outbreak.

"The recommendation right now is no," said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, adding that the complications of smallpox vaccine were greater than for those vaccinations that are still given in childhood. "And the reason those complications were accepted was because of the extraordinarily devastating impact that smallpox could have on society."

The Centers for Disease Control and Prevention, in Atlanta, has a reserve of roughly 15 million doses of smallpox vaccine. That vaccine, which is not available to the public, was manufactured using a method that dates from the 1700's. The method involves infecting calves with a related virus, vaccinia; the resulting pus is used in the making of the vaccine.

That process is considered barely acceptable for human vaccine today. So the government has contracted with a biotechnology company, Acambis, to make a similar vaccine using modern techniques that involve growing vaccinia in cells, and to deliver 54 million doses by next summer. Early next year, Dr. Fauci's institute will begin clinical trials of that vaccine; in the current climate, he predicted, there will be no shortage of volunteers.

At the same time, Tommy G. Thompson, secretary of health and human services, says he is negotiating with four drug makers to produce enough doses to cover every American.

The smallpox virus exists in laboratories that are overseen by the World Health Organization, although bioterrorism experts believe that clandestine stocks may exist. In any event, unlike anthrax, which is not transmitted from person to person, smallpox is highly contagious.

The smallpox vaccine can be used not only to prevent infection but also to treat people within a few days of exposure to the virus, and thus contain an epidemic.

An outbreak of smallpox in New York City in 1947 provides insight into how such a vaccination program would work, said Leonard A. Cole, who teaches science and public policy at Rutgers University and is the author of a book on biological warfare.

The outbreak, Mr. Cole said, involved a businessman who had been traveling in Mexico and arrived in New York on March 10 of that year. He died in a city hospital after infecting several others, but city health officials did not detect the outbreak until April 4.

The authorities then made a "crash decision to vaccinate everybody that had not been vaccinated since childhood," Mr. Cole said, but, with only 500,000 doses on hand, they soon ran out of vaccine. They turned to the military for help, and in three weeks 6.3 million New Yorkers had been immunized against smallpox. In the end, he said, 12 people were infected with the virus, and two died, including the first patient.

Mr. Cole described this method of responding as "a model that we know works." But today's population is far more vulnerable to smallpox: many Americans have never been vaccinated, and even those who have been now lack protection because the vaccine is thought to wear off after 15 or 20 years.

So some people say they would rather risk vaccination than the possibility of a slowly detected outbreak. Others are not so sure. "I'd want to know more about the side effects and the dangers to my daughter," said Elaine Herscher of San Francisco, the mother of a 5-year-old. "The real threat to her health from the vaccine could be far worse."

Even if the government does amass 300 million doses, it is unlikely to offer them without "substantial discussion," said Dr. Thomas V. Inglesby of the Center for Civilian Biodefense Studies at Johns Hopkins University.

Among the questions that would need to be answered: Who should be vaccinated first? Emergency workers? Health professionals? Children? The elderly?

In the meantime, the government is also paying for research into safer smallpox treatments. Dr. Mark Buller, a virologist at St. Louis University, is part of that effort. Because he studies viruses in the pox family, he is one of the few Americans who have recently been immunized against smallpox.

"I would not even consider having my family vaccinated," Dr. Buller said. "I feel I'm more likely to be hit riding my bike to work than to be hit by a smallpox episode in my own life."