Treatment of pregnant CBA/J females with CFA at day 0.5 and 7.5 of pregnancy significantly reduced the fetal resorption rates from 45% to 29% (P less than 0.05). Supernatants of progesterone-treated spleen cells from CFA treated CBA/J females pregnant of DBA/2 males significantly reduced natural cytotoxicity, while those of untreated identically pregnant mice had no effect. Supernatants of CFA-treated virgin mice blocked natural cytotoxicity to the same extent as those of CFA-treated pregnant mice. These data suggest that nonspecific immunostimulation induces progesterone receptors in spleen cells of CBA mice and that these receptors allow a progesterone dependent suppressive pathway to exert an antiresorptive effect.