Developing a new contraceptive for men may involve prompting an immune reaction to a protein that is produced in the male reproductive system.
The method worked in experiments on male monkeys and could one day supplement condoms and vasectomies, the current methods available to men.
When the immune treatment was stopped, most of the monkeys regained their fertility, researchers reported in Thursday's issue of the journal Science.
"Immunocontraception for males is a possibility and hopefully will be developed for human use over the next several years," said Dr. Michael O'Rand of the University of North Carolina.
In O'Rand's experiments, monkeys were immunized using a form of eppin, a protein produced in the testis and epididymis, the tightly coiled ducts that carry sperm.
Male monkeys that developed a strong immune response to eppin were able to copulate but could not impregnate females, the researchers said.
"We don't understand the exact mechanism yet, but we think the immunocontraception works by preventing the sperm from freeing itself from the seminal fluid to make its way to the uterus and oviducts to fertilize the egg," O'Rand said.
In the experiments, seven of the nine males tested developed high antibody levels. Five of the seven recovered fertility once the immunization stopped. They were injected with eppin about every three weeks to maintain the immunization.
Normally the body does not produce antibodies against its own proteins. But the testis and epididymis are protected by a barrier so the protein never gets into the bloodstream. When eppin was injected into the bloodstream, the immune system did not recognize it and produced antibodies.