When assayed by the ability to reduce oxidized glutathione to reduced glutathione, glucose-6-phosphate dehydrogenase deficiency of either the Negro or Caucasian mutant variety could be demonstrated in hemolysates only with hemoglobin concentrations below 2.0 Gm. per 100 ml. In intact erythrocytes, the inability to regenerate reduced glutathione was apparent regardless of the concentration of red cells. The process of hemolysis, therefore, appeared to permit the demonstration of higher levels of activity in G-6-PD deficient human erythrocytes than was possible in intact cells. A markedly deficient capacity to regenerate endogenous reduced glutathione or to reduce exogenous oxidized glutathione, however, could be demonstrated with the hemolysate of erythrocytes from a patient with hereditary nonspherocytic hemolytic anemia associated with a deficiency of glucose-6-phosphate dehydrogenase activity. These studies have emphasized the hazards involved in extrapolating the results of studies performed with hemolysates to metabolic processes within intact erythrocytes.