A patient with multiple intestinal fistulae maintained on total parenteral nutrition for 18 months developed low serum selenium. Erythrocyte glutathione peroxidase activity was 6% of normal. Erythrocytes were not able to metabolize H2O2 as well as those from controls, although the hexose monophosphate shunt itself was intact. Granulocytes from this patient had 15% of the erythrocyte glutathione peroxidase activity found in normals. Patient granulocytes were not able to metabolize H2O2 as well as controls, although the hexose monophosphate shunt was intact. Erythrocyte glutathione peroxidase-deficient granulocytes incubated with a respiratory burst stimulant, phorbol myristate acetate, had only 60% of the hexose monophosphate shunt activity present in control granulocytes. These abnormalities were reversed with selenium supplementation. Bacterial killing of Staphylococcus aureus 502A and cardiac function were not affected by selenium deficiency. Thus, selenium deficiency resulted in biochemical and functional abnormalities of erythrocytes and granulocytes. These abnormalities were reversed with selenium supplementation.