The primary IgM antibody response to sheep erythrocytes in vivo as well as in vitro is markedly decreased in the spleen cells of pregnant mice, compared to age-matched female controls. Decreased antibody synthesis appears to be mediated by nonspecific suppressor cells, because the addition of pregnant spleen cells to the normal spleen cell cultures causes a significant suppression of plaque-forming-cell responses of the normal spleen cells. Suppressor cell activity was not observed in lymph nodes of pregnant mice. At least two populations of pregnant spleen cells were shown to exert a suppressor cell activity; one is T lymphocytes and the other a nylon-adherent cell present in the B-cell-enriched macrophage-depleted fraction. Pregnant spleen cells cultured in vitro were shown to secrete a soluble suppressive factor(s) into the supernatant medium.