To evaluate the interaction of phenylbutazone with racemic warfarin or R,S-(±)-warfarin in man, S-(−)-warfarin or levowarfarin was synthesized with 13C label in the 2-position of the coumarin nucleus and added to [12C]R(+)-warfarin or dextrowarfarin to form a [12C/13C]pseudoracemate of warfarin. In six normal human subjects, a single oral dose of this “cold labeled” pseudoracemate, 1.5 mg/kg body weight, was administered with and without a daily dosage of phenylbutazone, 300 mg orally, beginning 3 d before the warfarin dose and continuing throughout the hypoprothrombinemia. Plasma samples were obtained daily and analyzed for warfarin content and for one-stage prothrombin activity. Unchanged warfarin in the plasma was fractionated by normal-phase, high-pressure liquid chromatography, and the enantiomorphic ratios were determined by chemical-ionization mass spectrometry with pentadeuteriowarfarin as the internal standard. A highly significant augmentation of the hypoprothrombinemia of the pseudoracemate occurred during the phenylbutazone regimen (P < 0.001) compared with pseudoracemic warfarin administered alone. There was a highly significant increase in the plasma clearance of dextrowarfarin (P < 0.01) and a significant decrease in the plasma clearance of levowarfarin (P < 0.05) during the phenylbutazone regimen compared with administration of warfarin alone. It was concluded that phenylbutazone augmented the hypoprothrombinemia of pseudoracemic warfarin stereoselectively by inhibiting the metabolic disposition of the more hypoprothrombinemic levowarfarin, yet reduced the plasma levels of pseudoracemic warfarin by greatly augmenting the metabolic disposition of dextrowarfarin.