- Embryonic loss prior to implantation appears to be a significantly frequent phenomenon and this is further reinforced by the very low pregnancy rates reported by in vitro fertilization and embryo transfer programs. Implantation failure may possibly be the result of rejection of the antigenic embryo by the hostile maternal immune system. The mechanism by which embryos in successful pregnancies escape these rejection responses may depend upon their ability to produce factor(s) that suppress in vitro mitogen-induced lymphocyte proliferation. Only 43% of cleaved embryos demonstrated this ability. We postulate that successful pregnancies are dependent upon the production by embryos of immunosuppressor factor(s) that has a direct suppressive effect on the maternal immune response.