Serum ferritin concentrations were measured during the first 6 months of life in 28 low birth weight infants (mean birth weight 1820 g range 900-2460; mean gestational age 34 weeks range 29-37) fed a standard formula fortified with ferrous sulphate. Fifteen of the infants received supplementary medicinal iron (ferrous succinate) from 3 weeks of age, and 13 only from 2 months of age. All were given vitamin E from 10 days of age. The serum ferritin values did not differ between the groups at 1-2 days, 8-10 weeks or at 6 months. Furthermore, there were no signs of hyperhaemolysis at 8-10 weeks in the group receiving medicinal iron early. The data indicate that the iron content in the formula is sufficient until 2 months of age, but also that thereis no disadvantage in starting medicinal iron at 3 weeks of age, if the diet is sufficient in vitamin E.