Research on the mechanisms of nonrejection of the fetoplacental allograft has focused on the tissues composing the fetomaternal interface, of which the placental trophoblast, the tissue directly confronting the maternal environment, is considered a prime candidate responsible for the survival of the fetus. We recently developed a method for isolating murine trophoblast, and found that a proportion of trophoblast cells from mature placentas, cultured for 2 days, express class I antigens on their surface, and this expression can be enhanced in vitro by interferon-alpha/beta (IFN-alpha/beta). In the present study, it was determined that cultured trophoblast cells from day 14 placentas were resistant to allospecific cytotoxic T lymphocytes (allo-CTL), while being susceptible to alloantibody and complement-mediated lysis. The trophoblast cells remained resistant to allo-CTL-mediated lysis despite IFN-mediated enhanced expression of class I H-2 antigens on their surface and the addition of phytohemagglutinin into the assay. Inhibition of protein synthesis also had no effect. In contrast, fetal fibroblasts, isolated from the same conceptuses, were readily susceptible to allo-CTL-mediated lysis. That the trophoblast cells do interact with the effector cells was shown by their ability to specifically inhibit the lysis of tumor target cells in a dose-dependent manner. Additionally, trophoblast culture supernatants did not inhibit the lytic activity of allo-CTL, even when concentrated 10- to 25-fold, indicating that a soluble suppressor factor was not inactivating the effector cells. These results suggest that trophoblast cells have a protein synthesis-independent mechanism of resistance to lysis by allo-CTL, which could play an important role in protecting the fetoplacental allograft from maternal immune rejection.