Researchers say they're a step closer to a contraceptive pill for men.
Scientists in Australia claim they've genetically modified mice to block two proteins needed in a male's reproductive system, but the mice were still have to have sex and had no other side effects.
In a study published today the researchers explained how they blocked the chemical message that tells muscles to move sperm cells, effectively making the male infertile.
This differs from other male contraceptive research which focuses on hormonal changes, which affects the male's sex drive.
The scientists say it's possible to recreate this genetic modification with chemicals, and a pill could be available in 10 years.