As a volunteer in the first human test of an experimental AIDS vaccine, Rusty Lynn thought he was ready for anything.

Doctors and nurses at the National Institutes of Health had spent hours making sure he understood what it meant to take part in one of the most closely watched trials in medical history. Lynn had signed a long consent form that warned he could develop serious, possibly life-threatening side effects from a vaccine that had never been tested in humans. He knew that after being injected he might test positive for AIDS and suffer the discrimination that people with the disease face.

What Lynn was unprepared for was the confusion and misinformation that swirled around his participation. Friends telephoned him and said they'd heard he was sick. Casual acquaintances shared conspiracy theories, suggesting that researchers had injected him with the actual virus -- not a part of the outer protein coat -- without telling him. A lifeguard at a local health club, alerted by an alarmed patron who had misunderstood a television interview Lynn had given, cornered him in the whirpool bath and bluntly asked: "Do you have AIDS?"

"It's made me realize how hard it is to do effective AIDS education," said Lynn, 45, a Presbyterian minister and social worker. "It also made me realize again how hard it is for people who do have AIDS."

Lynn, the second person to be injected with the experimental vaccine, is one of 44 volunteers enrolled in the initial phase of testing of gp160 -- the first such vaccine to be approved by the Food and Drug Administration for human testing. Like the other volunteers, he is a healthy gay man who repeatedly has tested negative for HIV, or human immunodeficiency virus, which causes acquired immune deficiency syndrome.

The first phase of the trial begun last September ultimately will involve 81 men -- 75 homosexual and six heterosexual -- and is designed to test the safety of the vaccine and its ability to produce antibodies that could fight the AIDS virus. Volunteers are injected with various doses of gp160, made from the outer coat of the virus that contains no infectious material, in the hope that the immune systems of healthy volunteers will produce antibodies that will attack the AIDS virus and block its ability to allow disease.

At no time does a volunteer come in contact with the AIDS virus; if he develops antibodies and tests positive, a process known as seroconversion, his blood will be placed in a test tube and exposed to the virus. As a control, one-fifth of the volunteers will receive a harmless protein instead of the vaccine.

Only a handful of drugs has been effective in delaying the infections that kill AIDS patients. None is a cure and none prevents infection with HIV as would a successful vaccine. Because of the extraordinary complexity of the AIDS virus, which cripples the body's immune system and leaves it unable to fight infection, scientists are divided on when -- or whether -- a vaccine will be available. Even if one is found soon, scientists believe it will not be commercially available before the mid-1990s.

"We labor under the presumption that we can and will find a vaccine," said Dr. Anthony S. Fauci, chief of the National Institute of Allergy and Infectious Diseases, the branch of NIH that is conducting the trial. Trials of gp160, short for glycoprotein with a molecular weight of 160,000, and several other vaccines are being conducted at other medical centers.

Last August, shortly after the Food and Drug Administration approved the test of the vaccine developed by MicroGeneSys, a Connecticut firm, and NIH researchers, NIH staff members began contacting doctors in the Washington area in an effort to recruit testing volunteers. Because of federal confidentiality statutes, NIH does not release the names of participants in drug trials.

For logistical reasons, officials had hoped that they could recruit enough volunteers from the Washington area. Although NIH received 550 calls, many prospective volunteers were found to have HIV or other health problems that rendered them ineligible.

Although NIH will furnish anyone testing positive with a notarized letter saying that the seroconversion resulted from the vaccine, some prospective participants dropped out.

"We're still recruiting and we need 20 more gay men," said Elaine Baldwin of NIAID, which has enrolled volunteers from as far away as Hawaii. So far, she said, no one has seroconverted or experienced serious side effects.

For gay men, who comprise nearly three-quarters of the nearly 54,000 AIDS cases in the United States, participating in the vaccine trial poses daunting dilemmas. Some volunteers are motivated by a combination of altruism, anger, survivor guilt and a pioneering spirit. They have watched friends and lovers die and have wrestled with the often agonizing decision about whether to get tested. Having learned they are not infected, they must be willing to risk jeopardizing their health to participate in the trial of a vaccine that not only may prove unsuccessful, but also could leave them with the permanent stigma of a positive test.

To Lance Clem, director of education programs for the San Diego AIDS Project, the potential gain is worth the risks.

"People look upon this as being heroic, but I've never seen it that way," said Clem, 35, who said he has lost 20 friends to AIDS and has worked with dozens of other people who are dying or dead. "I firmly believe it's only a matter of luck and chance that I haven't been infected. I can't sit here and tell friends who are infected to think positive and take care of themselves and then not do something to help."

Clem, who is due to be injected this week, plans to spend much of the next month commuting from San Diego to Bethesda for the weekly checkups required the first month of the trial.

NIH pays out-of-state volunteers air fare and $60 per day to cover food, lodging and other expenses. Clem said he will have to partially subsidize the trips himself.

Clem is no stranger to research medicine. In 1980, when he lived in Denver, he took part in a study of an experimental and now widely used vaccine against hepatitis B, a highly infectious blood-born virus that often strikes gay men and presents a particular risk to health care workers.

Another volunteer, William F. Wilson, 37, is a veteran of a Johns Hopkins University study of 1,200 uninfected gay men. Wilson, who works as a companion for AIDS patients under a program sponsored by the District, says he volunteered for the vaccine trial despite the objections of his lover.

"Life isn't without risks," said Wilson, who was injected in October. "Even if they scrap the whole thing tomorrow or a year from now discover they've been on the wrong track, it will have been worth it. At least they'll know more than when they started."

NIH officials have gone to unusual lengths to educate potential volunteers about the theoretical risks of gp160. All must pass a short oral quiz and must sign a five-page consent form furnished in advance and read before immunization by a physician. It says in part: "The investigators in this study can make no guarantee that the fact that you will have a positive antibody test as a result of immunization will not be used against you."

Lynn was well aware of that possibility when, on Sept. 21, he met with two doctors in a small office on the 11th floor of NIH's Clinical Center. First they answered his questions, and then one doctor read the consent form to him. He was warned that the vaccine could trigger a serious allergic reaction, impair his ability to respond to a subsequent, better vaccine, or cause the development of antibodies that might react with bone marrow cells, lower his white blood cell count and leave him with a damaged immune system.

Lynn said he understood all that, but still wanted to participate. As a precaution against anaphylactic shock, a potentially fatal allergic reaction, a nurse inserted an intravenous line into his right arm, then injected 10 micrograms, the smallest dose of the new vaccine, into his left arm. About 20 minutes later, Lynn recalled, he experienced a "funny taste" in his mouth. An hour later he went to work at Washington's Whitman-Walker Clinic, which provides social and medical services to AIDS patients.

For several months Lynn felt the same sort of nagging uncertainty that plagues those infected with HIV, who fear that every symptom could portend a major, possibly fatal, illness. In Lynn's case, every time he didn't feel well, he wondered whether he was having a vaccine reaction, and whether he was seroconverting and would then test positive for the rest of his life.

He worries less now, but said, "Until a couple of years have gone by, I'll probably have low-level anxieties."

Lynn said his work with 300 AIDS patients at Whitman-Walker has helped reinforce his belief that the decision to participate in the trial was right. "What overwhelms me several times a week," he said, "is that in spite of what we're trying to do here and how hard we work, people are still dying, still getting diagnosed, still getting exposed to the virus. This is a way for me to really make a lasting difference."