A "reversible vasectomy" form of male contraception has been successfully tested in monkeys.
It works by injecting Valsalgel into the vas deferens - the tube that carries sperm - where it forms an impenetrable gel barrier.
A study in rabbits last year showed that it had the potential to provide a reversible alternative to vasectomy, which involves cutting and sealing off the vas deferens.
The research showed that the gel could be removed by flushing the duct with baking soda solution.
In the new trial, Valsalgel prevented any conceptions occurring in a test group of 16 rhesus monkeys.
Lead scientist Dr Catherine VandeVoort, from California National Primate Research Centre, said: "Our research shows that Vasalgel placement into the vas deferens produces reliable contraception in mature male rhesus monkeys.
"Importantly, we show that the method of Vasalgel placement is safe and produced fewer complications than usually occur with a vasectomy."
Although it is possible to reverse a vasectomy, it is a technically challenging procedure and patients often have very low rates of fertility following reversal.
The findings, reported in the journal Basic and Clinical Andrology, pave the way for clinical trials expected to begin next year.
Allan Pacey, Professor of Andrology at the University of Sheffield, said that the idea "is not a new one" but that this progress is "a useful step in the right direction."