Scientists investigating the roots of the AIDS virus have found evidence of it in 12-year-old blood from Uganda, suggesting that some people are better able than others to fight off the usually fatal disorder.

While studying 75 blood samples still frozen since a 1973 study of lymphoma, researchers discovered that two-thirds of the samples contained antibodies for HTLV-III, the virus that is believed to cause acquired immune deficiency syndrome. The presence of AIDS antibodies in these people indicates that they had been infected by an ancestor of the AIDS virus -- or perhaps to the AIDS virus itself.

"And if it is the AIDS virus , why haven't these people had the disease?" says W. Carl Saxinger of the National Institutes of Health. "You'd expect at that level of exposure to see a lot of disease."

The level of antibodies is lower than that in AIDS victims, Saxinger says, indicating that the body had successfully fought off the disease. The blood samples were taken from people with an average age of 6 to 7 years.

Among the possible explanations:

* The Ugandans were exposed to an ancestor or "a close relative of the virus" that does not cause AIDS.

* Because they were exposed as children, they were better able to fight off the disease than adults are.

* They were infected in an as-yet unknown way -- not through sexual contact, as is commonly the case in the United States.

* The virus was, for some unknown reason, "pretty well tolerated by that population group."

The study confirms earlier theories that the disease originated in Africa, but research is needed to explain the Uganda findings, Saxinger says. "We're trying to locate historical serum collections that date back maybe even further."

Saxinger and his colleagues published their study in Science magazine.