WASHINGTON - 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.

Progress depends on funding for the work and, even with unlimited money, translating the findings into human use could take a decade, said O'Rand, lead researcher on the project.

"This is a proof of principle. It could be adapted for human use with continued development and the appropriate safety-toxicology tests," O'Rand said.

In recent years, researchers have developed some male contraceptives, based on hormones, that were designed to suppress sperm production. This work is now in trials.

O'Rand's experiments did not involve hormones. Instead, 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, designed in the United States and carried out in India, 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.

Dr. Patricia Anastasia DeLeon of the University of Delaware said the results were significant and that scientists were lucky to get a protein that would produce antibodies.

Normally the body does not produce antibodies against its own proteins, she said. But, DeLeon said, the testis and epididymis are protected by a barrier so the protein never gets into the bloodstream. So, when eppin was injected into the bloodstream the immune system did not recognize it and produced antibodies.

"I think it's significant because the reversibility is very attractive," said DeLeon, who was not part of the research team.

The findings are valid and interesting, said Douglas S. Colvard, associate director of CONRAD, a cooperative organization based at Eastern Virginia Medical School in Norfolk, Va. The group organizes and funds contraceptive development research, including O'Rand's project.

Colvard said that what needs to be done now is to repeat the experiment in other animals or in more monkeys.

Colvard also said researchers need to show that what happened in monkeys is likely to occur in humans, too.

Colvard said that while O'Rand's findings demonstrate a target for a potential male contraceptive, he believes hormone-based male contraceptives are closer to reaching the market. Indeed, he said, two major European drug companies are currently collaborating to develop such products.

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