Xanthopterin, a metabolic end product of the nonconjugated pterins dihydrobiopterin and tetrahydrobiopterin, is present in many organs and is known to inhibit the proliferation and growth of conconavalin-stimulated lymphocytes. We have developed a simple fluorometric method to measure xanthopterin in the blood and have validated the method by high pressure liquid chromatography (HPLC). Serum levels were 14 + 7 nmol/l in normal subjects and 141 ± 51 nmol/l in hemodialysis patients (p < 0.02). Intermediate levels from patients with renal insufficiency not on dialysis correlated with serum creatinine levels (p < 0.001). Xanthopterin (MW 179) was cleared by hemodialysis at a slightly lower rate than creatinine. It is bound to protein, but the binding, 90 ± 5% in normal subjects, is decreased in uremia to 60 ± 15%, p < 0.01. Red cell levels of xanthopterin were five times higher than those of plasma in normal subjects (69 ± 15 vs. 14 ± 7 nmol/l, p < 0.001), but uremic patients had lower levels in red cells than in plasma (101 ± 24 vs. 141 ± 51 nmol/l, p < 0.05). Slight or moderate hemolysis induced by mechanical stress increased plasma xanthopterin levels by 35 %, the effect being more pronounced when hemolysis was severe. We conclude that xanthopterin is increased and its binding to protein is decreased in chronic renal failure. The altered ratio of red cell/plasma xanthopterin levels may reflect an abnormality of the red cell membrane in uremia. We are conducting further studies to amplify our preliminary findings that xanthopterin inhibits cellular growth in