The plasma pyridoxal-5'-phosphate (PLP) level of alcoholic subjects has been compared with that of non-alcoholic individuals in order to ascertain the incidence of abnormal vitamin B(6) metabolism in chronic alcohol abuse. 66 alcoholic subjects were selected on the basis that they did not exhibit abnormal liver function tests and hematologic findings. 35 of them had plasma PLP concentrations less than 5 ng/ml, the lowest value encountered in 94 control subjects, indicating a high incidence of deranged PLP metabolism in alcoholic patients even when hepatic and hematologic abnormalities are absent. The biochemical basis for the altered PLP metabolism in chronic alcohol abuse was examined. Low plasma PLP levels in alcoholics were not accompanied by decreased pyridoxal kinase and pyridoxine phosphate oxidase activities in erythrocytes. Further studies with erythrocytes demonstrated that the cellular content of PLP is determined not only by the activities of these PLP-synthesizing enzymes but also by the activity of a phosphate-sensitive, membrane-associated, neutral phosphatase, which hydrolyzes phosphorylated B(6) compounds.Acetaldehyde, but not ethanol, impaired the net formation of PLP from pyridoxal, pyridoxine, and pyridoxine phosphate by erythrocytes. However, when the B(6)-phosphate phosphatase activity was inhibited by 80 mM phosphate, this effect of acetaldehyde was abolished. By the use of broken cell preparations, it was possible to demonstrate directly that the action of acetaldehyde is mediated by the phosphatase, resulting in an acceleration of the degradation of the phosphorylated B(6) compounds in erythrocytes.