Aldehyde dehydrogenase, glucose-6-phosphate dehydrogenase, and pyruvate kinase activities were determined in erythrocytes of various ages, separated by Percoll gradient centrifugation, in 13 alcoholic patients and eight control subjects. The total erythrocyte activities of all three enzymes were not affected by alcoholism, however, the youngest cells of alcoholics had a decreased aldehyde dehydrogenase activity, while both glucose-6-phosphate dehydrogenase and pyruvate kinase activities were increased. The depression of aldehyde dehydrogenase activity not only persisted, but became more marked after 2 weeks of abstinence, while the enhanced activities of the two other enzymes returned to normal. These observations suggest that chronic alcohol ingestion suppresses aldehyde dehydrogenase in the bone marrow, while it enhances other erythrocytic enzymes.