Erythrocyte catalase activity has been found to be decreased in iron-deficiency anemia. This is the first demonstration of deficiency of an iron-containing enzyme in iron-deficiency anemia in humans. It was measured by the method of Takahara et al and expressed as catalase activity per 10⁹ red blood cells. In all treated patients, catalase values returned to normal after the anemia was corrected. In patients with anemia due to other causes, the erythrocyte catalase activities were normal.