Adult male inbred Wistar rats were given a subtoxic dose of cadmium (25 ppm) orally for 4 weeks prior to and during immunization with sheep erythrocytes (E). The control group received tap water during the experimental period. No difference in the antibody response to E was found between the two groups. Mononuclear peripheral blood cells from experimental and control rats were stimulated with concanavalin A (Con A). The experimental rats had received 25 ppm Cd in the drinking water from weaning until sacrifice at 15 weeks of age. There was no difference in [3H]thymidine incorporation between the cells of the experimental group and those of the control group. Addition of Cd at very low concentrations to the cultures significantly depressed the [3H]thymidine incorporation of the cells from both groups of rats. Thus, a subtoxic dose of Cd apparently had no effect on the in vivo immune response in rats.