Iron supplementation is recommended for formula-fed infants, and it is currently suggested that iron be supplemented in formulas at a minimum of 6 mg/L. A negative side effect of iron supplementation, however, may be a reduction in the absorption of elements with similar physicochemical properties. We investigated this possibility by evaluating the impact of iron supplementation at 6 mg/L on tissue concentrations of zinc, copper, and manganese. Weanling mice fed a supplemented diet for 4 weeks had markedly lower tissue manganese concentrations than mice fed a nonsupplemented diet. Tissue zinc and copper levels were only marginally affected. The potential negative effect of iron supplementation on manganese status in the human infant should be evaluated.