ABSTRACT: Cell dynamics in the thymus and spleen of mice during the first allogeneic pregnancy and during postpartum were investigated. A considerable decrease in the number of thymocytes occurred at midpregnancy and was sustained until postpartum. There was an increase in the number of splenic T cells on the third postpartum day. These splenic T cells were shown to have the Lyt-1+ 2− phenotype, as determined by multiparameter analysis, and their responsiveness to Con A was increased compared with splenic T cells of virgin mice. Since such T cells were not observed in the postpartum spleen of adult thymectomized mice, they may have derived from the thymus. The response to Con A by normal virgin spleen cells was depressed when spleen cells from 3-day postpartum mice were added to the culture as the third party cells. Thus, the increased number of splenic T cells may act as inducer cells in the suppressor circuit during postpartum.