Studies were conducted to determine the period of immune system development which was most sensitive to perturbation by the known immunotoxicant di-n-octyltin dichloride (DOTC). Fischer 344 rats were exposed prenatally, both pre- and postnatally or postnatally to DOTC by oral gavage of pregnant and/or lactating females. At various ages, ranging from 3 to 16 weeks of age, offspring were examined for a number of immune functions. These included: body and lymphoid organ weights; lymphoproliferative responses to B and T cell mitogens; natural killer cell activity; and primary antibody response to sheep erythrocytes. Prenatal (day 10-20 of gestation), pre- and postnatal (day 11-20 of gestation and 2-11 days of age), or postnatal (2-13 days of age) oral dosing of dams to 20-50 mg/kg DOTC resulted in no consistent alteration in immune function in offspring. These results suggest that direct dosing of pups during early postnatal life may be the most effective means of inducing immunosuppression with DOTC during immune system development. The results also provide direct evidence for the greater sensitivity of the developing immune system compared with the fully developed immune system for a known immunotoxicant. (Copyright (c) 1988 Hemisphere Publishing Corporation.)