A virus that causes leukemia has become the leading candidate for the cause of the deadly acquired immune deficiency syndrome (AIDS), according to Dr. Myron Essex, chairman of the Department of Cancer Biology at the Harvard School of Public Health.

In a report to be published in the Sept. 9 issue of Science magazine, researchers from five universities and the Centers for Disease Control add important evidence to the belief that AIDS may be caused by a virus called the Human T-Cell Leukemia Virus, or HTLV.

"HTLV is a leading candidate, the leading candidate, partly because no other agent has held up as a serious candidate for very long, and also because all the knowledge we have about HTLV fits the pattern of AIDS so far . . . , " said Essex, lead author of the new report.

Bruce Evatt of the Centers for Disease Control said that HTLV is now the agent on which most CDC attention is intensely focused.

Alone, the new data does not lay out a clear case that HTLV is the sole cause of AIDS, and other candidates still may crop up, but this report is one more piece of evidence pointing to some variant of that virus.

Most people have not been exposed to HTLV. But it appears from several tests that more than half of AIDS victims have been exposed to the virus. Researchers say it is possible that far more than half of AIDS victims have been exposed, but the test is too difficult to interpret to be sure of this.

Since some groups of people are at higher risk of AIDS than others, the researchers say that if HTLV were the cause of AIDS they would expect to find many people in those high-risk groups who have been exposed to HTLV.

In the new study, 172 persons with the blood disease hemophilia who do not have AIDS were tested; 12 percent were found to have been exposed to HTLV. By comparison, 1 percent or less of normally healthy people show any sign of having been exposed to the virus.

Hemophiliacs are at high risk of contracting AIDS, and the presence of HTLV in their bodies is exactly the result that would be expected if HTLV were the cause of AIDS.

In addition:

* For almost two years it has been suspected that AIDS is caused by an infectious agent, probably a virus, transmitted by very close contact between persons and through body fluids such as blood and semen. HTLV fits with those suspicions.

* AIDS attacks the immune system, rendering it helpless when infections strike, and AIDS victims generally die of one of many possible infections they contract after their immune response is destroyed.

HTLV attacks one cell specifically in the body, the "T-helper" cell of the immune system, a cell so crucial to immune response that if the T-helpers are killed off, immune response is virtually destroyed. T-cells are blood cells that help mount the body's attack on intruding germs. HTLV damages the T-cells when it causes cancer, and Essex said he knows of no other virus that attacks T-cells specifically.

"So we hypothesize that AIDS may be caused by a variant of HTLV, somewhat different than the HTLV that causes leukemia," Essex said.

* The agent causing AIDS would be expected to be found in high levels in victims of the disease, and in people likely to become victims. So far, other infectious agents have not been found consistently in victims and high-risk populations. HTLV now has been.

* AIDS is believed to be a new disease, and such diseases mainly spring up as variants of longer-known infectious agents. It also happens that the longer-known variety of HTLV has been found in elevated levels in Japan and in the Caribbean, two places that now have also turned up cases of AIDS.

The CDC has reported that AIDS had infected 2,157 persons in the United States as of Aug. 22, and so far 852 of them have died. None has been cured, so the percentage of victims who eventually will succumb is uncertain.

The disease was first reported in the summer of 1981, and four groups gradually have emerged as having a high risk of contracting the disease: sexually active homosexual men account for most of the victims; intravenous drug users, hemophiliacs and some Haitians also have been victims in disproportionate numbers.

One alternate explanation of the research results so far is that another virus, related to HTLV or merely structurally similar without being directly related, has shown up as a chemical imposter of HTLV in tests for the virus.

It is also possible that the reason for some of the results is that HTLV is just another infection that attacks AIDS victims when their immune system is already impaired, and so is a result--not a cause--of the disease.