The selenium (Se) concentration in plasma, serum and whole blood was determined in 29 healthy persons, 19 nondialyzed patients with chronic renal disease (9 proteinuric but with otherwise normal renal function, 10 with chronic renal failure), and 10 dialyzed end-stage renal disease patients. No differences in Se concentrations of plasma, serum or whole blood were determined between patients and their matched controls. Total Se concentrations did not correlate with any of the Se carriers except with α2-globulin. However, α2-globulin concentrations were not different between patients and controls. Within the dialyzed patient group, Se concentrations did not change during dialysis. Se supplementation is not indicated in these patients. Within all groups analyzed, serum Se concentrations (12.7 ± 2.5 μg/dl) were significantly (p < 0.0001) lower than comparable plasma concentrations (15.3 ± 3.9 μg/dl). These data suggest normal concentrations of blood Se in the groups of patients evaluated; and they are not Se-deficient. Serum Se concentrations cannot be equated with plasma v