Calcium significantly diminishes the absorption of ferrous and ferric iron in a dose-related manner, whether the test doses of calcium and radioiron are administered orally or introduced into isolated intestinal segments; the effect is maximal in the duodenum and jejunum. Electron microscopic observations and quantitative studies of the mucosal uptake and transfer of iron show that calcium decreases the entry of iron into the microvilli of intestinal epithelial cells. Animals fed a high-calcium, iron-replete diet developed iron depletion; animals consuming high-calcium food of marginal iron content developed mild iron deficiency anemia. Further, more radioiron was absorbed from human milk than either bovine milk or human milk supplemented with calcium. These data suggest that individuals consuming a high-calcium diet contain marginal amounts of iron could develop iron deficiency anemia and may explain why infants who are fed cow's milk have a greater incidence of iron deficiency anemia than those fed human milk.