The binding of [14C]warfarin to normal human serum albumin (HSA), carbamoylated albumin, and plasma of patients with renal failure was investigated. Warfarin binding to extensively carbamoylated albumin (exhibiting a molar ratio of homocitrulline: albumin of ca. 7) was markedly diminished. The decrease in binding was apparently due to a decrease in the primary binding affinity constant. The Ka for warfarin binding to HSA was 1.6 × 105 M-1 compared to 0.88 × 105 M-1 for carbamoylated HSA. Binding to normal plasma was significantly greater than to plasma from uremic patients. Mean fractions bound (±S.E.M.) were 98.0 ± 0.2 and 94.1 ± 1.6 respectively (P < 0.02). The extent of carbamoylation of albumin from patients with renal failure, however, was low, exhibiting a homocitrulline: albumin molar ratio of 0.27. The binding of [14C]warfarin to carbamoylated albumin with a homocitrulline: albumin molar ratio of 0.5 was marginally diminished, suggesting that the carbamoylation of albumin that occurs during renal failure contributes only slightly to the defective plasma binding of warfarin in uremia.