Like an estimated 1.2 million other Americans, Bill, a healthy 34-year-old federal government analyst, has been exposed to the virus that causes AIDS. Unlike most of them, he knows it.

A blood test that Bill took in July revealed the presence of antibodies to HTLV-3, the AIDS virus. Designed to screen the nation's blood supply, the test also is widely available to determine whether people in high-risk groups -- particularly sexually active gay men -- have encountered the virus.

A negative test result is strong evidence that one has not been exposed and therefore will not get AIDS.

The meaning of a positive result is less clear. Scientists estimate that at most one out of 10 people exposed to the virus will get AIDS within five years. They don't know which ones, they don't know when and they don't know why.

People who test positive are left mainly with doubts.

"One looks at one's values," says Bill, who may be an AIDS carrier but has no symptoms. "Life is very transient. I could be sitting in an aircraft with 30 minutes to write notes to loved ones before we crash.

"Or I may have something lurking inside me which five years from now may manifest itself."

Such doubts have led to a variety of reactions: In the District, personal ads have begun to appear in The Washington Blade, the city's gay newspaper, describing the advertiser to prospective dates as "HTLV-3 negative." In San Francisco, most gay men who at first said they would take the blood test have, on second thought, decided not to, according to one survey. "As the test became a reality," says researcher Stephen F. Morin of the University of California at San Francisco, "the psychological conflicts became more real." In Paris, two gay men reportedly committed suicide in June after learning their blood tested positive for the virus. Doctors treating AIDS patients acknowledge the reports but decline to discuss specifics, citing patient confidentiality.

Fears such as these -- and the psychological trauma associated with them -- have prompted Washington's Whitman-Walker Clinic and some other gay-oriented health services to advise people not to take the test.

"For the most part, the gay population is not taking the test," says Jason Whiddon, head of Whitman-Walker's HTLV-3 antibody testing program. "I think a lot of them just don't want to know, and we're not discouraging them from not knowing."

But since recent publicity about AIDS, including the news that actor Rock Hudson has the disease, the clinic's Wednesday night screening sessions have been filled to their 50-person capacity.

Whiddon says that regardless of how the test comes out, people should take the same steps to prevent the spread of AIDS. "The results are of intellectual interest at best," he says. "No matter what the results, anyone at high risk should be practicing safe sex."

"Safe sex" means avoiding the exchange of body fluids -- especially semen and blood, and to a lesser extent saliva -- which are implicated in the transmission of AIDS. Safe sex practices also help prevent contact with other viruses that scientists believe may join forces with the AIDS virus and trigger the severe disease.

But some other experts don't believe people should avoid the blood test.

"Lack of information really doesn't help anybody," says Thomas Coates, a psychologist at the University of California at San Francisco. He has counseled about 20 people who carry the virus.

"People have the responsibility to know, because it has implications for how they take care of themselves," says Coates, who is involved in a major study of gay men in San Francisco.

Knowing one carries the virus, Coates says, can be the jolt needed to modify sexual habits and to protect the immune system by generally taking better care of oneself.

"When someone finds out they're antibody positive," Coates says, "what it does is personalize the epidemic and make them feel very vulnerable."

He says that after a patient learns he has been exposed, he usually reacts with a short period of emotional crisis, then enters a two- to three-month period of celibacy, followed by a slow, careful reentry into sexual relationships.

Such awareness is viewed by some as the best way to stop the spread of AIDS.

"This is the real way one is going to control it -- to be aware who's at risk" of transmitting the virus, says Richard Decker, manager of AIDS research at Abbott Laboratories in North Chicago, the leading manufacturer of the antibody test. "Some people can accept facts and live with them. Other people can be so concerned by the information that they have it, it will destroy their life."

Once people know whether they are carriers, he says, "what they do about it -- well, we can't offer any advice." People who go to Whitman-Walker in Adams-Morgan to take the free test first must attend a session explaining what the test means and how it works. It can take from two to six months to develop antibodies after exposure to the virus, experts say. If someone tests negative, "he may have been exposed and just hasn't developed the antibodies yet," Whiddon says. An estimated 2 to 3 percent may never develop antibodies, and it is unknown whether those people will get AIDS.

If the blood tests positive or questionable, the sample is checked again.

By this time, results are usually clearly negative or clearly positive, Whiddon says. But if doubt remains, the test is sent to Abbott for a more sophisticated and expensive test.

People must come into the clinic to receive their results from a trained counselor.

"A lot of people cry," Whiddon says.

Those who test positive sometimes ask whether the results were a "strong" positive or a "weak" positive. Scientists speculate that if AIDS is like some other viral diseases, such as hepatitis B, those with a strong positive may be the ones most likely to get AIDS, while those with a weak positive may be able to fight it off, says Abbott Laboratories' Decker. "But we don't know when it's going to change" from weak to strong, he says.

Nor do scientists know why some people get no symptoms, others develop AIDS-related complex (ARC), a set of symptoms including fatigue and weight loss, and others die of AIDS. Of those people who test positive, an estimated 20 to 25 percent will develop ARC, which sometimes, but not always, leads to AIDS. Scientists don't know how people who have been exposed to the virus can avoid getting AIDS, a reason cited by men in the San Francisco study for not taking the test.

Some researchers hope to answer that question.

"We're looking at how the test or other pieces of information might change people's attitudes and behaviors, preventing it from going to ARC or AIDS," says psychologist Lydia Temoshok of the University of California at San Francisco.

Temoshok and other researchers, funded by the National Institute of Mental Health, studied AIDS patients -- and some people who had the virus but no symptoms -- in London and Paris, comparing their coping abilities with those of San Francisco patients.

In London, members of a support group called The Body Positive believed taking the blood test "was critical to helping them change their behavior." Changes have included practicing safe sex, forming "closer relationships based on other than physical attraction," and in general trying to maintain good health.

"Other infections or stress may push you along the course," Temoshok says.

But she also concedes that knowing one carries the virus can itself be a source of stress. Bill, the government analyst, knew two people who died of AIDS. For him, the stress is worst part. When he went to Whitman-Walker, he was confident he had not been exposed and was seeking reassurance; he had been following safe-sex guidelines for two years.

"I walked into the test session with an 'Oh no, not me' attitude," he says. "Now I just try to put it in its proper place. I am very rigid about safe-sex rules. I try to eat a more balanced diet. I don't stay out until 3 in the morning."

But despite the changes in his own life style, he says the "fear and paranoia" from his positive result leads him to advise others against the test. "I see no need for the test to be taken by the average gay person," he says.

And there's also the question of the future and how to think about it.

When he took the blood test in July, Bill says, "I was debating taking out an IRA for this year. Part of me was saying enjoy the money now -- don't wait for 65."

A few weeks later, he went to a bank and bought the IRA.