Circumcision can reduce the rate of HIV infections among heterosexual men by around 60%, a study suggests.
Cells under the foreskin are vulnerable to infection
The South African study, reported in Public Library of Science Medicine, found it had a protective effect for some of the 3,280 young men involved.
Circumcision is thought to help protect against HIV because cells under the foreskin are vulnerable to the virus.
UK experts warned some circumcised men in the study still became infected and condoms offered the best protection.
HIV infection rates are lower among groups in Africa who practise circumcision, but it was not known if this was due to cultural differences.
When the foreskin is removed, the skin on the head of the penis becomes less sensitive and so less likely to bleed, thereby reducing the risk of infection.
Studies in Uganda and in Kenya are also investigating the link.
The South African trial, conducted by a team of French and South African researchers and sponsored by ANRS (the French National Agency of Research on Aids), took place in the Orange Farm area near Johannesburg, where male circumcision in adulthood is a common but not universal practice.
Just under 3,280 young, sexually active, uncircumcised, heterosexual men who took part in the study were offered the chance to be circumcised and then monitored for HIV infection.
Just under half chose to be circumcised.
The researchers planned to test all participants for HIV at three, 12 and 21 months, to see whether there was a difference in the rate of new infections between the two groups.
However, after 18 months, the number of new HIV infections in the control group was 49, compared with 20 in the treatment group.
The researchers decided at this point it would be unethical to continue the study.
It was stopped and the uncircumcised men were offered circumcision.
UNAids has said the trial found promising results, but more work needs to be done to confirm its findings and "whether or not the results have more general application."
'Not a condom substitute'
Keith Alcorn, of the National Aids Manual, said: "Although this study showed that men who were circumcised were less likely to become infected with HIV, it must be stressed that circumcised men did become infected in this study, and that circumcision does not provide total protection against HIV.
"I don't think that any country will be moving towards promotion of circumcision for HIV prevention on these results alone.
"Two further studies in Kenya and Uganda have yet to be completed, and will give us more information."
Deborah Jack, chief executive of the National Aids Trust, added: "There is a danger that people who have been circumcised will feel that they are fully protected from HIV when they are not.
"We need more research and clear guidance, as circumcision can never be a substitute for condom use."