Although serious hemorrhage during therapeutic coadministration of sulfinpyrazone and racemic warfarin occurs, no prospective studies have been done. In this study, single oral doses of racemic warfarin, 1.5 mg/kg, were administered to six normal subjects with and without oral sulfinpyrazone, 400 mg daily. Both the hypoprothrombinemia (p less than 0.001) and the plasma warfarin concentrations (p less than 0.05) were significantly augmented. To determine if this interaction was stereoselective, the experiments were repeated in the same subjects with R-and S-warfarin enantiomorphs. S-warfarin with sulfinpyrazone caused a highly significant augmentation of both the hypoprothrombinemia (p less than 0.001) and the plasma warfarin concentrations (p less than 0.001). R-warfarin with sulfinpyrazone did not significantly change the hypoprothrombinemia but significantly (p less than 0.05) reduced warfarin concentrations. Thus, sulfinpyrazone augmented the hypoprothrombinemia of racemic warfarin stereoselectively by reduced metabolic clearance of S-warfarin. Sulfinpyrazone and racemic warfarin are most dangerous when either drug is added to a stabilized regimen of the other drug.