Treatment of female F-344 rats and C57B1/6 mice with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during the latter half of gestation and in the postnatal period resulted in a severe depletion of lymphocytes in the thymic cortex of the offspring. Cellular immunity was impaired in these animals. Allograft rejection times were prolonged in rats and mice. Graft-versus-host (GVH) activity of spleen cells as well as the response of rat thymus and spleen cells to phytohemagglutinin (PHA) was reduced on a cell-for-cell basis. No reduction occurred in the response of thymus cells to concanavalin A (Con A). Maternal treatment postnatally on day 0, 7 and 14 with 5 μg TCDD/kg of body weight reduced total PHA and Con A response recoverable from the thymus by 91 and 74%, respectively. Histologically, depletion of thymic cortex was not due to lymphocyte destruction. There was no adrenal hypertrophy. Stress-induced release of glucocorticoids is not considered responsible for the immune suppression observed. In contrast to the marked effects in early life, reduced responsiveness of spleen cells to PHA in mice treated with TCDD when 1 month old was only seen at a dose level that was clearly toxic. In 4-month-old mice, PHA responsiveness as well as GVH activity were not decreased. Response of unstimulated and stimulated thymus and spleen cell cultures was not reduced by adding TCDD in vitro. In TCDD-exposed rats, decreased eosinophilia was observed in acidophilic cells in the adenohypophysis. This finding is discussed in view of the effect of TCDD on the thymus, and a possible mode of action of TCDD is proposed.