SAN FRANCISCO, JUNE 21 -- One of the world's most famous AIDS researchers said Wednesday that he is no longer convinced that the human immunodeficiency virus (HIV) is the sole cause of AIDS. His statement challenged one of the fundamental scientific beliefs about the disease.

In an unusual three-hour session Wednesday night at the Sixth International Conference on AIDS here, Luc Montagnier, the French co-discoverer of HIV, presented data showing that a bacteria-like organism known as a mycoplasma spurs a slowly reproducing population of AIDS viruses into one that multiplies much faster -- at least in cells growing in a dish. He suggested that in humans, mycoplasma infection turns an otherwise benign HIV infection into a disease.

Montagnier's statement immediately drew criticism from a wide spectrum of other AIDS experts. They said it appeared to challenge a large body of research accumulated since the AIDS virus was discovered seven years ago.

In another report, scientists presented the strongest evidence yet that progress of an HIV infection can be dramatically slowed if the patient is put on the drug AZT at the earliest stage of infection -- long before any symptoms arise.

Margaret Fischl reported that her studies at the University of Miami showed that the benefit of AZT is greater the earlier it is given in the infection.

Mycoplasma infections, which cause a number of diseases including pneumonia, have been thought for some time to be one of several infections that attack the body along with HIV, weakening the immune system further and possibly explaining why some people become ill faster than others.

But in a conclusion that drew skepticism from many AIDS researchers, Montagnier took the mycoplasma hypothesis further, saying that he found it "quite feasible" that the microbe may be the critical additional factor that turns HIV from a "peaceful" virus into a killer. "This is not to say that HIV is not the primary agent, but there are serious shortcomings in the idea that it causes all the disease," Montagnier said.

Even mycoplasma specialists, who have been arguing for some time that the organism plays an important role in the disease, said Montagnier had taken an unusually bold position based only on preliminary data.

"My feeling is that there will have to be a whole lot more work done before we can understand this," said Joseph G. Tully, a mycoplasmologist at the National Institutes of Health. "I don't know if I would go as far as he has at this point."

"He has everything to lose if he's wrong," said Joel Baseman, chairman of the microbiology department at the University of Texas at San Antonio.

Montagnier's hypothesis addresses one of the mysteries of AIDS, namely how a virus that lies quietly in the cells of infected patients for years can suddenly explode and begin killing the cells of the immune system.

Montagnier said that if he added mycoplasmas to a dish of HIV-infected cells in the laboratory, the viruses began reproducing much faster. He also said that when he added an antibiotic that kills mycoplasmas, the rate of virus proliferation dropped to a low level. The antibiotic has no effect on HIV.

Together, Montagnier said, these findings suggest that the mycoplasma has a synergistic effect with the HIV virus to make it lethal, and that HIV alone is necessary but not sufficient to cause AIDS.

Montagnier's doubters said a number of other infectious agents -- including cytomegalovirus, adenovirus and herpes virus -- have been shown to increase potency of HIV in lab dishes. But none have any proved impact on the course of the disease in humans.

Still, the fact that the idea has been advanced by a scientist as prominent as Montagnier has created a stir here, adding a note of scientific controversy to a meeting that has had few surprises.

"I was impressed by his openness and willingness to extend the data," Baseman said. "But I'm not sure my comfort zone is the same as his comfort zone."