A Dutch study of homosexual men infected with the AIDS virus has provided the first strong evidence that the drug AZT could protect some of those who are infected but have not yet become ill.

Earlier research had shown that the drug can improve and prolong the lives of people with AIDS, but this study of 18 infected men with no visible symptoms suggests that AZT can also shield the immune system from damage caused by the virus.

"This does not mean everyone should immediately use the drug," said Dr. Jaap Goudsmit, professor of virology at the University of Amsterdam, where the research was conducted. "But the results are striking, and they show great promise."

"This is a major finding and we are glad to see it, but it should not be misinterpreted," said Dr. Samuel Broder, chief of clinical oncology at the National Cancer Institute and a developer of AZT and other AIDS drugs. "It is not a cure. And it is not a big enough study to answer all our questions," he added.

Because the study lasted only six months and the AIDS virus can lie dormant in the body for years, scientists still do not know whether AZT will keep infected people from getting sicker over a long time. And nobody knows the long-term side effects of the powerful drug.

But the report, in this week's issue of the British medical journal The Lancet, shows that 14 of the 18 men who took the drug had significant declines of a protein that is a key part of the AIDS virus. The amount of that protein is considered a measure of the severity of infection.

In addition, CD4 cells, which play a critical role in the immune system's defense, increased in 14 of the 18 subjects. By comparison, only one of seven untreated subjects had improved CD4 counts and none of them had declines in the amount of viral proteins, called antigens, in their blood.

Several of those treated with AZT also had swollen glands, which is often an early sign of HIV infection. The researchers reported a "striking" disappearance of swollen glands for the people on the drug but not for those who weren't. None of the participants in either group developed AIDS during the study.

In the United States, more than 1 million people are believed to be infected with the HIV virus, which causes AIDS. Over the past two years researchers have worked with increasing urgency to develop a drug that would prevent carriers of the virus from getting sick. Many efforts are under way, but AZT, which is the only drug that has been approved for use by AIDS patients, has held out the greatest hope.

The National Institutes of Health has begun a three-year test of AZT on infected people who have no symptoms of AIDS. So far more than 1,000 people have enrolled in the trial, in which half the participants will get a placebo and half will get AZT.

Researchers say that it is essential to conduct a trial of that size and duration to assess the benefits of AZT more accurately.

To multiply, the AIDS virus needs thymine, one of the four building blocks of DNA. AZT, or azidothymidine, mimics the structure of thymine and attaches itself to the viral DNA chain, ending it prematurely. As a result, the virus can no longer multiply.

"The really important question is: Can this drug prevent AIDS in otherwise healthy people who are infected with the virus?" said Dr. Paul Volberding, chief of AIDS activities at San Francisco General Hospital. "That is something that only time can tell."

He and others said the findings are exciting because they give reason to continue the controlled trials initiated last year by the NIH. If the Dutch study had indicated that those who took AZT did no better than those who did not, the large American trials would have been thrown into doubt.

One of the problems AIDS patients have had with the drug was that it had to be taken in high doses four times a day. The high doses cause severe anemia in many people. Almost half of those taking it have had to discontinue use.

But the Dutch study lowered the dosage and the frequency and found that side effects were infrequent and mild. Only two subjects developed anemia.