French and South African AIDS researchers have called an early halt to a study of adult male circumcision to reduce HIV infection after initial results reportedly showed that men who had the procedure dramatically lowered their risk of contracting the virus.

The study's preliminary results, disclosed Tuesday by the Wall Street Journal, showed that circumcision reduced the risk of contracting HIV by 70 percent -- a level of protection far better than the 30 percent risk reduction set as a target for an AIDS vaccine.

According to the newspaper account, the study under way in Orange Farm township, South Africa, was stopped because the results were so favorable. It was deemed unethical to continue the trial after an early peek at data showed that the uncircumcised men were so much more likely to become infected.

All of the men in the study had been followed for a year, and half the men had been followed for the full 21 months called for in the original study design, according to the Wall Street Journal, which obtained a draft copy of the study.

Begun in August 2002, the experiment is one of three closely watched clinical trials in Africa to determine whether there is scientific merit to nearly three dozen less rigorously controlled studies showing that circumcised men were much less likely to become HIV-positive.

The hope is that, lacking a vaccine, the nearly 5 million new HIV infections occurring each year could be slowed by circumcision, the surgical removal of the foreskin -- a simple, low-cost and permanent medical intervention that is a common but controversial cultural practice in much of the world. In Africa, about 70 percent of men are circumcised at birth or during rite-of-passage ceremonies in early puberty.

Medical anthropologists began noticing as early as 1989 that the highest rates of HIV infection in Africa were occurring in regions of the continent where the predominant tribal or religious cultures did not practice circumcision. Adult HIV infection rates above 30 percent are found in Zimbabwe, Botswana, Swaziland and eastern South Africa, where circumcision is not practiced; yet HIV infection rates remain below 5 percent in West Africa and other parts of the continent where circumcision is commonplace.

Laboratory studies have found that the foreskin is rich in white blood cells, which are favored targets of HIV, the virus that causes AIDS. So the theory is that men who are uncircumcised are much more likely to contract the virus during sex with an infected woman, and that the epidemic spreads when these newly infected men have sex with other women within their network of sexual partners.

The lead investigators of the study, Dr. Bertran Auvert of the University of Paris and Adrian Puren of South Africa's National Institute for Communicable Diseases, are not talking. The results were expected to be discussed at an AIDS conference in Rio de Janeiro in three weeks. But word about the findings has been circulating among researchers searching for ways to slow the epidemic.

"I would be thrilled if it works, but we will also need the results of other trials,'' said Johns Hopkins University epidemiologist Ronald Gray, who is conducting, in Uganda, one of two other controlled clinical trials of male circumcision.

Gray's trial, which has completed enrollment of 5,000 men in the Rakai district of Uganda, is not scheduled to end until 2007.

A third trial, under way in Kisumu, Kenya, is still enrolling its quota of 2,700 volunteers and is also expected to be completed in 2007, according to the National Institute of Allergy and Infectious Diseases, which is sponsoring it.

All three trials were designed to compare the HIV infection rates of two groups of HIV-negative men, one-half of whom would agree to be circumcised, the other to be offered only counseling on AIDS prevention. The studies were designed to show whether or not circumcision provided a statistically significant protective effect of at least 50 percent.

The South African study -- if the results are confirmed -- suggests that the level of protection afforded is even higher.

Although the apparent protective effect of circumcision has been noted for more than 20 years, doubts linger as to whether circumcision itself is protective, or whether the lower risk may be the result of cultural practices among those who circumcise. HIV rates are low in Muslim communities, for example, which practice male circumcision but also engage in ritual washing before sex and frown on promiscuity.