The variety of infant diets given subjects in this group during the first year all provide iron in amounts of 0.5 mg/kg/day or more except in one of the 59 children. This level of dietary iron intake was adequate to meet iron requirements for hemoglobin synthesis and to prevent development of hematologic or clinical evidence of iron deficiency. Food intake adequate to support rapid growth rates contains enough iron to support the needed relative acceleration of hemoglobin synthesis. Lower intakes of iron are associated with higher percentage utilization in hemoglobin synthesis and higher intakes result in lower utilization. Supplementation of diets with iron in several forms occurred in 25% of the group without hematologic evidence of response as compared with the group dependent on dietary iron alone.