The purpose of this study was to compare concentrations of vitamin B6 compounds and the activities of enzymes that synthesize or catabolize pyridoxal 5'-phosphate in the plasma and erythrocytes of nonalcoholic and alcoholic subjects. Blood was obtained from male nonalcoholics and chronic alcoholics with minimal liver damage and normal hematology. Plasma, erythrocyte, and urinary B6 compounds were analyzed by high performance liquid chromatography, and pyridoxal phosphate was also measured enzymatically. Erythrocyte pyridoxine kinase and pyridoxine phosphate oxidase and erythrocyte and plasma pyridoxine phosphate phosphatases were assayed. Plasma pyridoxal phosphate concentration was significantly lower in the alcoholics (31.3 +/- 3.6 nmol/liter) than in the nonalcoholics (58.7 +/- 7.5 nmol/liter). The concentrations of the other B6 compounds in plasma, erythrocytes, and urine were not different in the two groups. Plasma alkaline pyridoxine phosphate phosphatase activity was significantly higher in the alcoholics (4.05 +/- 0.36 nmol/(h.mg] than in the nonalcoholics (3.01 +/- 0.18 nmol/(h.mg]. The activities of erythrocyte kinase, oxidase, and phosphatases were not significantly different in the two groups. The relationship of plasma pyridoxal phosphate concentration to its metabolites and the activities of the enzymes involved in its metabolism was determined. Plasma pyridoxine phosphate phosphatase activity assayed at pH 9.0 or 7.4 correlated negatively with plasma pyridoxal phosphate concentration. The low pyridoxal phosphate concentration observed in the plasma of the alcoholic subjects may in part be related to increased plasma phosphatase activity.