Federal officials said yesterday that the first AIDS vaccine approved for human trials in the United States has shown "very encouraging" results in laboratory animals, causing them to produce an immune defense reaction and antibodies that halted the action of the AIDS virus in test tubes.

The officials said the impressive animal results and the ability of MicroGeneSys Inc. to complete all the necessary tests quickly were among the reasons the West Haven, Conn., company was given approval by the Food and Drug Administration to conduct the first tests. Other vaccine candidates are expected to be approved in the near future.

"This is the first step in what will be a long process toward developing a vaccine to prevent AIDS," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease. "It will be a considerable time, probably the mid-1990s, before any vaccine, including this one, will be ready for general use."

Several other groups have sought FDA approval for vaccine candidates, including a group headed by Allan Goldstein of George Washington University, a group at Oncogen Inc. in Seattle and researchers at Viral Technologies Inc.

Drugs have been developed that can stall the advance of acquired immune deficiency syndrome. But they are not cures and cannot prevent infection with the AIDS virus, which attacks the body's immune system, rendering it incapable of resisting other diseases and infections.

The testing of volunteers is scheduled to start in October at the National Institutes of Health. Fauci said 60 male homosexuals and three heterosexuals not infected with the AIDS virus will be given the vaccine in tests that will last about six months. The men will be tested twice at the beginning of the experiment to be sure they are not infected.

As a control group, another 18 volunteers will be given a mock vaccine for comparison.

The volunteers will be monitored to see if the vaccine has any side effects common to vaccines -- pain, swelling, soreness at the vaccination spot or, worst of all, a severe allergic reaction to the inoculation.

The volunteers will be monitored to see if they produce antibodies and if those antibodies, when put in a test tube, can protect cells against infection by the virus, said Thomas Folks, an NIH researcher who helped create the vaccine in collaboration with MicroGeneSys.

In what are called "phase one" trials, the vaccine will be tested for safety only; researchers hope to see no significant side effects.

If the initial tests go well, the "phase two" tests will judge safety in 200 or so volunteers. It is not until "phase three" tests, which will involve thousands of volunteers, that effectiveness will be officially tested.

But even in the early trials, researchers will look for some signs of whether volunteers develop two kinds of immune system reactions.

In one form of reaction, the body reacts to invaders such as bacteria and viruses by making antibodies that attach themselves to the invaders, fouling their ability to cause disease.

The idea of the AIDS vaccine is to take a piece of the AIDS virus -- just the outer protein coat, so there is no danger of AIDS infection -- and inject it into people, hoping their bodies will mistake the protein coat alone for the whole AIDS virus. Their immune systems would then mount an antibody attack on the protein coat.

If these antibodies, when they are produced by the inoculation, are the right type and are made in sufficient amounts, they might be able to prevent infection with the virus.

The second form of immunity that the researchers hope to see eventually is "cell-mediated." In this type of immunity, "killer cells" are stimulated to seek out and destroy the invader.

In studies of two chimpanzees, both antibodies and cell-mediated immunity was seen, though not at very high levels, according to NIH and MicroGeneSys officials.

It is not clear what meaning these results have, since chimpanzees infected with the AIDS virus do not react like humans, and do not get AIDS.

Homosexual males are one of the groups at highest risk of contracting AIDS. Fauci said they will be asked to volunteer because homosexuals eventually will be prime candidates for a vaccine if one is developed.

The volunteers, who must agree to follow safe sexual practices and other AIDS-avoidance behavior during the trial, will be given various doses of the vaccine and some will get booster injections later, researchers said.