ASHINGTON, Oct. 21 — As the federal government starts negotiating to procure enough smallpox vaccine to inoculate every American, a separate effort has already begun to obtain large quantities of the scarce medicine needed to treat people with adverse reactions to the vaccine, federal health officials said today.

Federal rules say the medicine, a colorless liquid known as vaccinia immune globulin, must be on hand before smallpox vaccinations may be administered. The medicine can be obtained only from people recently inoculated against smallpox, and existing American reserves may have deteriorated.

Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, said in an interview today that the federal government had a small quantity of the medicine in stockpiles and had begun contacting medical companies to ask them to check their supplies.

The American population appears to be more vulnerable to dangerous smallpox vaccine reactions now than it was when civilians stopped being vaccinated in 1972, Dr. Fauci and other experts said.

Even when given to healthy people, smallpox vaccines infrequently produce dangerous reactions because the vaccine uses a live vaccinia virus, which is similar to smallpox. Vaccinia can cause large, oozing lesions spreading from the site of the vaccine, especially in people with immune system problems or severe skin problems. It can be life-threatening. A large but unknown number of Americans now have immune systems that are either depressed because of H.I.V., the virus that causes AIDS, or have been chemically suppressed during treatments for cancer, organ transplants, severe burns, autoimmune diseases like lupus, and other health problems.

When Army doctors administered a routine smallpox vaccine in the mid-1980's to a soldier in basic training who seemed healthy but was actually developing AIDS, his body became covered with large sores and he nearly died. Saving him required all of the vaccinia immune globulin in a large Army medical depot, said Dr. D. Craig Wright, a former military doctor who treated the soldier.

The Food and Drug Administration said two years ago that the nation's small stockpile of vaccinia immune globulin had mysteriously turned pink, and barred its use. An agency official said today that the agency had concluded that the pink liquid could be used only as an "investigational new drug" under close medical supervision.

The agency is also trying to obtain fresh vaccinia immune globulin, an official said.

Vaccinia immune globulin can only be harvested from the blood of people who have been inoculated against smallpox and have developed antibodies against the disease, experts said. The United States stopped vaccinating civilians against the disease in 1972, and stopped vaccinating soldiers in the early 1990's. Only a small number of people, mainly lab workers, have been inoculated since then.

Dr. Fauci said federal authorities had asked companies to check supplies of other globulin for whether it might be usable. Globulin from people vaccinated in the military in the early 1990's might be effective, he said.

Pharmaceutical companies will begin producing new batches of smallpox vaccine within the next several months, and some of this vaccine will need to be tested in people for safety and for whether it causes them to produce antibodies. Participants in these clinical trials will be asked to donate vaccinia immune globulin, Dr. Fauci said.

Roughly 1 in 4,000 healthy recipients of the vaccine encounter serious side effects that clearly require treatment with vaccinia immune globulin.

Federal agencies have also begun contacting medical groups to estimate the number of Americans with immune-system difficulties who will not be able to receive smallpox vaccines, Dr. Fauci said; some doctors have guessed that there are a million such Americans. If there is a smallpox outbreak and vaccinations begin, these people will be urged to take extra care not only in avoiding smallpox-infected people but also in avoiding any physical contact with the sites of other people's vaccinations.