RU-486 (17 f8-hydroxy 4-dimethylaminophenyl-17-a-propenyl estrone 4,9 diene-3-one; mifepristone) is suggested to act by binding to progesterone and glucocorticoid receptors. Based on its chemical nature, we anticipated that RU-486 may have potent antioxidant properties. We used the oxidation of LDL as our model system. RU-486 and a similar compound, onapristone, at 1-5-IuM concentrations, decreased the formation of oxidized LDL. LDL isolated from plasma of subjects who were orally supplemented with RU-486 was resistant to oxidation, as compared to LDL isolated from control plasma.The antioxidant effect of RU-486 appears to reside in the dimethylaminophenyl side chain moiety. Reduction of the A-ring of the steroid molecule had no effect on its antioxidant property. Analogs of RU-486 which lack the dimethylaminophenyl group, were without antioxidant activity. Levoniorgestrel, which