The effects of TCDD exposure on the developing immune system were investigated in F344 rats. Fetal and neonatal rats were exposed to TCDD through maternal dosing (5 μg/kg) on day 18 of gestation and on days 0, 7, and 14 of postnatal life (group I). Another group of neonatal rats was exposed to TCDD through maternal dosing on days 0, 7, and 14 of postnatal life only (group II). Body weights and thymus/body weight ratios were found to be suppressed up to 145 days of age in group I, but only up to 39 days of age in group II. Parameters of cell‐mediated and humoral immune function were investigated. TCDD suppressed cell‐mediated immune function without affecting humoral immune function. Suppression of T‐cell function was selective in that “helper” cell function was not suppressed.