A class of well-known antibiotics has unexpectedly been found to block a step that helps the AIDS virus reproduce, researchers report. But they caution that the antibiotics have not yet been tested in people, that they have dangerous side effects and that many candidate drugs that show promise in the laboratory fail in the clinic.

Nevertheless, the researchers believe that they may have found a new approach to designing drugs to fight HIV, the virus that causes AIDS.

The drugs are the family of antibiotics known as aminoglycosides, which kill bacteria by preventing them from making proteins. In a paper published in the current issue of the journal Cell, Dr. Michael Green of the University of Massachusetts and his colleagues report that the drugs also adhere to RNA molecules of the AIDS virus and thwart viral infections in cultured white blood cells.

The most effective of these aminoglycosides is neomycin, which attaches itself to the viral RNA as if tailor-made for it. But, Green warned, neomycin itself is too toxic for patients. He said he hoped to devise chemical modifications that would be less toxic but still deadly to HIV.

Green cautioned that because neomycin could have serious side effects and because the experiments had only been conducted in the laboratory, people with AIDS should not treat themselves with the antibiotic. He said the research was at least two years away from even small studies in humans.

Dr. Martin Hirsch, director of AIDS research at the Massachusetts General Hospital in Boston, echoed Green’s warning. “Neomycin just is not going to be of clinical use for this indication,” he said. “It is currently used only for bowel cleaning prior to certain operations. It is very toxic; it is toxic to the kidneys.

“The more important thing this paper does is to open a new avenue for investigation. Hopefully, Green or someone else will find a drug with a similar structure that is less toxic and that might make it into clinical trials.”

But Green said he was encouraged by the results so far. “The reason we’re very excited is that we think this provides a new start to design antiviral drugs,” he said.