A battery of sex steroids were used to check their effect on mouse melanoma (B16F10) cell growth in-vitro. Progesterone and its synthetic receptor antagonist RU-486 showed maximum inhibition on in-vitro melanoma cell growth [1]. Further research work showed that the inhibition by progesterone was not a toxic, spurious or non-specific effect on mouse melanoma cell growth and the inhibition by progesterone was not mediated through progesterone receptor.