The activity of glutathione peroxidase (GSH Px), glucose-6-phosphate dehydrogenase (G-6-PD), hexokinase, and glutamic oxaloacetic transaminase (EGOT) was measured in 78 blood samples. GSH Px activity was not found to correlate with hexokinase or EGOT activity, indicating that it was not a strongly age-dependent enzyme. Although modest elevations of GSH Px activity were observed in the red cells of patients with a variety of hematologic disorders, the most consistent and striking increases in activity were observed in G-6-PD-deficient subjects.