The prospect of a unisex contraceptive -- a birth control pill for both men and women -- is being raised by California biologists.It will take years, however, to prove that the natural hormone they discovered will work the way they suspect.

Preliminary evidence indicates that the hormone, called inhibin, is normally produced in the ovaries and testes. Released into the bloodstream, it travels to the pituitary, the master gland regulating the reproductive organs.

Under the influence of inhibin, the pituitary will not produce another hormone (called follicle-stimulating hormone) that tells ovaries and testes to make mature eggs and sperm. The researchers speculate that people given extra doses of synthetic inhibin would be rendered temporarily sterile.

An advantage of inhibin is that it allows the pituitary to continue sending out a third hormone (leutenizing hormone) that regulates the manufacture of sex hormones by the testes and ovaries.

The research, published in Nature, the British scientific journal, was done by a team at the Salk Institute in La Jolla headed by Nobel laureate Roger Guillemin and a collaborating team at Genentech Inc., a commercial genetic engineering firm in South San Francisco headed by Peter Seeburg.

Most details of inhibin's natural role remain unknown. Male and female reproductive systems are regulated by a complex web of chemical signals shuttling among several glands and other organs, and inhibin's role may turn out to be far less manipulatable than expected.

Genentech researchers are working to synthesize enough inhibin to test on animals as a contraceptive. Research so far has been done on pigs but all indications are that most mammals, including human beings, use similar hormonal systems.