Chemicals such as 2,3,7,8-tetrachlorodibenzodioxin (TCDD) adversely affect immunologic function in offspring following treatment of pregnant mice late in gestation. TCDD or diethylstilbestrol (DES), another immunotoxicant, are administered to pregnant C57BL/6 mice at various times during gestation to: 1) establish the sensitive period for induction of immunologic deficits and 2) identify the initial lesion. T and B lymphocytes, and subpopulations of these cells, are immunocytochemically stained to determine the effects of these chemicals on immune system development. Quantitation is by flow cytometry. Studies to explore the inter-relationships of developmental immunotoxicity with the induction of structural malformations, weight gain, and transplacental carcinogenesis are planned.