Small amounts of milk do inhibit ferrous iron absorption from a 5 mg 59Fe2+ dose in 1- to 18-month-old infants. Only 50 ml of 2/3 cow milk reduced the absorption from 18+/-3% (Xa +/- S.E.) to 3.8 +/- 1.2% in infants with normal iron stores (inhibition index 0.21) and from 26 +/- 3% to 8.5 +/- 1.4% in infants with depleted iron stores (inhibition index 0.33%). Milk does not inhibit the bioavailability of hemoglobin iron. From a 5 mg dose of hemoglobin-59Fe added to 50 ml of 2/3 cow milk 4.8 +/- 1.0% were absorbed by infants with normal iron stores and 8.3 +/- 0.8% by infants with depleted iron stores. The low iron content of milk (50 mug Fe/100 ml) and its poor bioavailability (approximately5% in infants with normal iron stores) would require a dialy consumption of 32 1 of unfortified milk to cover infants daily iron requirement of 0.8 mg/day. The supplementation of 2--3 milk meals per day with 5 mg hemoglobin iron each meets the whole iron requirements of infants with depleted and normal iron stores respectively and can be used for iron prophylaxis in infancy during the first and second year. Prophylaxis with inorganic iron requires an empty stomach and duodenum for optimal bioavailability. A daily dose of only 5 mg ferrous sulfate iron is enough to cover the total iron requirement of infants.