- Mus caroli blastocysts surgically transferred to the Mus musculus uterus implant and develop successfully during the first 9.5 days of gestation. At this time, the trophoblast giant cell area becomes infiltrated with lymphocytes and, subsequently, massive hemorrhage occurs, associated with embryo death. This process is specific for the xenogeneic embryos and does not affect adjacent allogeneic embryos present in the same uterus. Cells taken from the resorbing tissues contain a population of large Thy 1+, Lyt 1+, Lyt 2+ cytotoxic cells capable of lysing Mus caroli concanavalin A blasts in vitro. It is suggested that failure of the Mus caroli embryos in the Mus musculus uterus is mediated by the maternal immune system and that the xenogeneic blastocyst transfer system provides a suitable animal model system for studies of mechanisms that prevent rejection of the fetal allograft.