Jejunal biopsies were obtained from five iron-deficient infants 24 to 32 hours after an intramuscular injection of iron-dextran. Newly produced cells at the base of the villus appeared to have a higher cytochrome-oxidase activity compared to the older cells at the tip which were produced during a period of iron deficiency. These results in man, similar to those previously described in the rat, suggest that the repair of intestinal cytochrome-oxidase activity which accompanies treatment of iron deficiency is limited by the rate of new cell production. In the iron-deficient rat histochemical evidence of cytochrome-oxidase repair corresponds closely to the extent of migration of new cells up the villus as indicated by autoradiography, between 8 and 48 hours following simultaneous administration of tritiated thymidine and iron-dextran. Colchicine, which arrests cell division at metaphase, interferes with cytochrome-oxidase repair when administered simultaneously with iron-dextran. Enzymes not known to contain iron or require it as a cofactor were assayed in jejunal scrapings from iron-deficient and control rats. Aspartate transcarbamylase, alkaline phosphatase, sucrase, and maltase activities were normal in the intestinal mucosa of the iron-deficient rat. No intestinal malfunction was demonstrated in the iron-deficient rat by determination of fecal fat excretion, I131 PVP excretion, and transport of glucose and galactose in everted sacs of the jejunum and the ileum.