Food and Drug Administration Commissioner Frank E. Young challenged the results of clinical trials of the AIDS drug ribavirin yesterday, saying "regrettably, we can find no evidence of effectiveness."

The announcement came at a presentation of ribavirin results before scientists here attending the Third International Conference on AIDS.

Few drugs are being tested on humans for treatment of AIDS, but researchers are developing many others. Dozens of tentative findings involving new drugs and how they respond to HIV infections are being scrutinized this week at the conference.

Last week, Young told a congressional hearing that he was suspicious of the initial test results on the drug, manufactured by ICN Pharmaceuticals of Costa Mesa, Calif. He said he doubted that they resulted from random testing.

Dr. Peter Mansell of Houston's Institute for Immunological Disorders, who conducted some of the tests, replied that the results were available for anyone to study. Mansell reported that the drug had showed promise in delaying the progression from HIV infection to AIDS in 163 homosexual men.

Scientists yesterday also reported indications that AZT, the only drug approved so far for AIDS treatment, has benefited patients with Kaposi's sarcoma and that it leads to improvement in neurological function among some of those infected with the AIDS virus.

Despite questions about ribavirin, a number of promising drugs were described at yesterday's meetings. Some, including dideoxycytidine, ddC, are beginning clinical trials.

"We have no idea whether {ddC} will work," said Dr. Samuel Broder of the National Cancer Institute. "But the drug is on track."

Twenty patients have been treated with ddC, a chemical cousin of AZT, and none has shown toxic side-effects.

Broder said the immune system of some patients showed improvement, but he cautioned that this cannot be considered proof of effectiveness.

Broder declined to predict when larger human trials would begin on ddC's ability to prolong life, but both the University of Miami and Stanford University are gathering patients for such a study.

Dr. Grace H.W. Wong of Genentech Inc., a South San Francisco biotechnology company, presented studies showing that a combination of tumor necrosis factor (TNF) and interferon can block AIDS virus infections and kill cells infected with the virus.

A clinical trial is under way at San Francisco General Hospital to determine the appropriate doses of interferon and TNF in AIDS patients, but no data is available.

Later this week, researchers from HEM Research Inc. of Rockville and Hahneman University in Philadelphia will publish in the British medical journal Lancet the results of an early clinical trial of Ampligen, a synthetic ribonucleic acid (RNA) that induces interferon production in the body.

A number of other drugs are being developed but are not ready for clinical trials.

Researchers from Harvard University described a novel approach to blocking the AIDS virus. Using castanospermine, an extract from the sap of the Australian chestnut tree, Dr. Bruce Walker, Dr. William Haseltine and others showed that it is possible to block a cellular enzyme required to coat the virus with sugar molecules.

By changing the way the virus is coated with sugar molecules, the researchers were able to prevent the AIDS virus from infecting cells and to block the virus in cells it already had infected. The drug is not ready for clinical trials.

A number of AZT-like drugs have been shown to protect against the AIDS virus in cells growing in laboratories. All of these drugs block the production of viral genes.

Another group at the National Cancer Institute has developed a different approach of blocking the virus' genetic machinery by using chunks of synthetic genes. Though the material seems to block the replication of the virus, it is too early to tell whether it can be used therapeutically.

The NCI's Broder noted that scientists have made much progress in the past two years. "We do not have a cure yet, but we are well on the way," he said.