Methemoglobin reductase activity (MHR), measured by the Hegesh assay, decreases as normal red cells age. This probably accounts for the slighlty increased concentration of methemoglobin in older red cells. Exaggeration of normal MHR age lability was demonstrated in several individuals with various molecular abnormalities of NADH-diaphorase and was associated with more striking accumulation of methemoglobin in the older red cells of those patients. Diaphorase activity, measured by the Scott assay, does not exhibit age lability in normal red cells or the cells of affected patients. While diaphorase uses either NADH or NADPH as a cofactor, methemoglobin reductase requires NADH. Methemoglobin reductase is found in hemolysates but not in membrane preparations, while approximately 1% of total red cell NADH-diaphorase activity is membrane-associated. The specific activity of NADH-diaphorase is approximately 60 times greater than that of methemoglobin reductase. Whether the two activities represent distinct enzyme proteins or differing substrate affinities of the same enzyme awaits adaptation of the Hegesh assay to enzyme electrophoresis.