The bioavailability of iron in dried whole egg, ground beef, whole-wheat flour, enriched flour and FeSO4 was evaluated in terms of hemoglobin regeneration. Sixty weanling male rats were made anemic by feeding a semi-purified basal diet containing 7.8 p.p.m. iron. They were divided into six groups to be fed the basal diet or the basal diet substituted with appropriate amounts of each iron source for 3 weeks. It was calculated that the milligrams of iron converted to hemoglobin per 100 mg of iron consumed was 51 for the iron in FeSO4 diet, 45 for the iron in egg diet, 43 for the iron in beef diet, 33 for the iron in whole-wheat flour diet and 24 for the iron in enriched flour diet. The efficiency of converting iron in FeSO4 diets to hemoglobin iron is linear over a wide range of iron intakes between experiments and among laboratories.