To evaluate the interaction of disulfiram with the separated enantiomorphs of racemic warfarin, seven normal subjects received single doses of R-warfarin (1.5 mg/kg body weight) and S-warfarin (0.75 mg/kg body weight) with and without a daily dose of disulfiram 250 mg, beginning 3 days before the warfarin dose and continuing for the duration of the hypoprothrombinemia. Disulfiram augmented the S-warfarin hypoprothrombinemia (p less than 0.001) but not that of R-warfarin (p less than 0.10). Disulfiram did not alter plasma concentrations of either R-warfarin (p greater than 0.10) or S-warfarin (p greater than 0.40). Disulfiram augments the hypoprothrombinemia of racemic warfarin stereoselectively by interacting primarily with S-warfarin. As disulfiram did not change the plasma concentrations of either enantiomorph, it may augment the anticoagulant effect of racemic warfarin by directly affecting the hepatic mechanism responsible for the hypoprothrombinemia.