Thirty-six infants who were exclusively breast-fed were observed for 9 months. Thirty-two infants who were completely weaned prior to age 3 1/2 months served as controls; these infants received iron supplementation in formula and solid foods. A great majority of exclusively breast-fed infants were able to maintain their iron status at the same level as that of the control infants. The mean concentration of hemoglobin was higher in breast-fed infants than in control infants at ages 4 and 6 months. However, six breast-fed infants required iron medication because they had laboratory evidence of iron deficiency, although none had anemia. Maternal iron supplementation during breast-feeding, even in large daily doses, did not have any effect on the infants' iron nutrition, nor prevent infants from developing some signs of iron deficiency. Our data indicate that it is safe in exclusively breast-fed infants to shift the starting age for introduction of iron to 6 months.