AbstractProgeny from benzo(a)pyrene (BP) mposed (150 ug/g body weight) primiparous mothers, injected during the second trimester of pregnancy, are severely compromised immunologically. In view of maternal-fetal associations, we studied, during pregnancy and postpartum, the quantitative and functional status of the maternal T cell population in the thymus and/or spleen. In the thymus, there is an exacerbated depression in the amounts of thymocytes, (tH+, Lyt 1+, Lyt 2+cells) during pregnancy relative to the corn oil-injected controls, which is sustained postpartum. In the spleen, while there are inconsistencies in the level ofθ+cells, The Lyt 1+are depressed postpartum relative to virgins, but the Lyt 2+are enhanced during pregnancy and postpartum, reaching levels<700-fold of controls. In controls, while the number of Lyt 1+cells was higher than BP-exposed mice or virgins during pregnancy, they virtually disappeared postpartum. A similar but reversed image is mirrored by the Lyt 2+cells, i.e., they were virtually absent during pregnancy but increased postpartum. Splenic allogeneic and syngeneic mixed lymphocyte responses were subnormal. The data show that BP disrupts the maternal T cell repertoire, leading to an accumulation of Lyt 1--2+cells, and suggest that splenic disruption may be due to changes in the differentiation potential of T precursor cells. These changes not only are most likely to affect maternal defenses, but also may have a direct bearing on the establishment of progeny immune status.