Rats were fat diets containing 6, 12, or 250 ppm iron throughout gestation and lactation. On day 17, pups immunized with sRBC were used to determine antibody synthesis by the Jerne plaque assay. In both iron-deficient groups, antibody formation was decreased by at least 50% compared to controls. For 3 weeks beginning on day 21, iron-deficient pups were fed either a control diet (35 ppm iron) or the same iron-deficient diet as fed to the dam. IgG and IgM formation was only slightly improved in repleted rats and remained significantly below that of rats fed the control diet throughout the experiment. In contrast, 250 ppm iron pups fed an iron-deficient diet postweaning had significantly decreased IgG and IgM production compared to littermates fed a control diet postweaning. Maternal iron deficiency during the critical pre- and postnatal growth periods may result in long-term impairment of humoral immunity that is not corrected by dietary iron repletion after weaning.