Lymphocytes from 5 individuals were cultured to study the effect of mestranol and norethindrone on sister chromatid exchange (SCE), blast transformation, mitotic index and micronuclei formation. Four combinations of the drugs were compared to water and ethanol controls. Some steroid combinations increased the SCE frequency compared to ethanol controls. There was no uniform effect on blast transformation or mitotic index. In 3 individuals, micronuclei formation was reduced in treated cultures. In conclusion, contraceptive steroids may be mutagenic in some individuals.