2,4-Dichlorophenol (DCP) is a drinking and waste-water contaminant formed by the spontaneous reaction of chlorine with phenols following chlorination of water for disinfection and deodorization. Rats were exposed to 0, 3, 30, or 300 ppm DCP in drinking water either in utero or for 12 wk postnatally following in utero exposure. Toxicity to DCP was assessed by organ and body weight changes, histopathology, and effects on reproduction and immunocompetence. Reproductive parameters measured included conception, litter size, pup birth weight, number stillborn, survival to weaning, and weaning weight. Immune parameters assessed were humoral immunity (antibody production) by an indirect enzyme-linked immunosorbent assay (ELISA), cell-mediated immunity by a delayed-type hypersensitivity response, and macrophage function by phagocytosis of radiolabeled blood cells. Rats that received the combined in utero and postnatal treatment with 300 ppm DCP had significantly increased liver and spleen weights, enhanced humoral immune responsiveness, and depressed cell-mediated immunity. Histopathologic changes were unremarkable in DCP-exposed rats, even in the presence of increased liver and spleen weights. The 6-wk-old progeny of DCP-treated dams had normal immune functions and showed no signs of DCP toxicity, other than increased spleen weights in the 300-ppm exposure group. The results indicate that (1) the immune system may be a sensitive target for chlorinated phenolic compounds, (2) DCP may exert different effects on separate major immune responses, and (3) unlike some other chlorinated phenols, DCP does not appear to alter reproductive performance in rats.