A French scientist reported here today that he has found a drug that blocks reproduction of the AIDS virus in patients who are already infected.

The drug does not cure acquired immune deficiency syndrome, but preliminary findings suggest that, at least in the small group tested so far, it may slow or halt the often-fatal disease. The drug also produces potentially fatal side effects if not withdrawn soon enough.

The drug, a chemical developed 10 years ago as an anticancer agent called HPA 23, joins two other experimental drugs, suramin and ribovarin, that U.S. researchers are either testing or plan to test as treatments for AIDS.

The HPA 23 findings were reported by Jean Claude Chermann of the Pasteur Institute in Paris, where the AIDS virus was isolated in May 1983, a year before Robert Gallo of the National Institutes of Health reported isolating an AIDS virus. The two viruses have turned out to be nearly identical.

Chermann spoke at a seminar sponsored by the Scientists Institute for Public Information and the AIDS Medical Foundation.

Chermann said the first patient to receive the new drug, a 13-year-old hemophiliac boy, is believed to have contracted AIDS from a blood transfusion. He was given HPA 23 until the chief side effect, a decrease in blood platelets that can lead to uncontrollable internal bleeding, became worrisome. During that time doctors were unable to find any AIDS virus in his blood.

When the platelet count improved, the drug was given again, then stopped in December 1983. Since then no AIDS virus has shown up in the boy's blood. Chermann said the child has returned to school and seems to be doing well.

Chermann said the results encouraged them to try the drug on 33 more patients. In all 33, virus proliferation stopped. But 11 died anyway, apparently because the disease had progressed too far before treatment began.

Controversy emerged at the seminar over whether to issue a major warning to heterosexuals about the dangers of AIDS. In the central African country of Zaire, AIDS is transmitted predominantly among heterosexuals, a situation that is beginning to show up in the United States.

Mervyn Silverman, former head of the San Francisco health department, suggested that AIDS among heterosexuals today may be at the same point as among homosexuals five years ago. "We may be wishing in five years that we had done more in 1985 to educate the public at large," Silverman said.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases and an AIDS researcher, emphasized that there were still very few cases among heterosexuals but said there is no question that the disease can spread among heterosexuals. "Am I worried about it?" Fauci said. "Yes."