We investigated the effect of prolonged iron supplementation of low birthweight infants on plasma zinc concentrations. Thirty infants (neonatal weight 820-2000 g) were examined at 6 or 12 months of life to assess their iron state and plasma zinc concentration. All of them were feeding on iron fortified formulas, which supply about 1 mg/kg/day of iron. In addition, 14 of them had been receiving 2 mg/kg/day of medicinal ferrous salts for at least five months. The other 16 were not given medicinal iron supplementation. Whereas iron state significantly differed in the two groups, plasma zinc concentrations were similar. Moreover, no zinc values below 75 micrograms/100 ml were found in any case. The results suggest that long term iron supplementation of low birthweight infants, at the recommended doses, does not influence zinc plasma concentrations unfavourably.