Virus researchers have found evidence that could help vindicate Robert Gallo's controversial claim to have discovered the AIDS virus independently of a rival French group.

Gallo's claim has been dogged by one main question: Why was the virus he said he discovered at the National Institutes of Health so genetically similar to that found by his French competitor, Luc Montagnier of the Pasteur Institute?

Most isolates of the AIDS virus that come from different patients differ by more than 6 percent in their genetic sequence and sometimes as much as 20 percent because of the virus's extraordinary ability to mutate.

But Gallo's virus was less than 2 percent different from Montagnier's.

That raised the contention that Gallo had made his discovery in a test tube contaminated by a sample of the French virus strain that had been sent to Gallo's lab.

Now, in papers in the August issue of the Journal of Virology, two researchers report finding two almost identical strains of the virus from two unrelated AIDS patients.

If confirmed, this would show that separate isolates of the AIDS virus do not always have to be so genetically different.

But there is a question mark.

As it turns out, one of the researchers, Mario Stevenson of the University of Nebraska, used to work in the Columbia University laboratory of the other researcher, David Volsky, where the two men jointly developed cell lines used to grow isolates of the AIDS virus.

Given the similarity between the two isolates, the question has arisen whether the genetic similarity is the result of a contamination that somehow traveled from one lab to the other.

Efforts are underway to track down both patients from whom the virus isolates were taken and to obtain new isolates.

Should they truly be similar, it may lift the cloud of suspicion hanging over the foremost AIDS researcher in the United States.

Alternatively, the study could show how easy it is for contaminations to happen quite by accident.