If Sprague-Dawley rats, 25-30 days old, are fed a diet containing 4-5 mg% of Mg, about 25% of survivors develop a large tumor of the thymus within 6-12 weeks. The tumor is composed of lymphoblasts, which seem to arise from the thymic reticuloendothelial system and, at times, disseminate as an acute T cell lymphoma-leukemia of unknown etiology. If the tumor cells are transmitted intraperitoneally to rats, 14-16 days pregnant, a local invasive and generalized disease is established in the mother but not in the fetuses or their domain. However, if the neoplastic cells are injected into the fetal domain, they colonize the fetal tissues. The colonization by tumor cells is most impressive in the extravascular structure of the placental labyrinth but not in the placental syncytiotrophoblastic zone at the maternal-placental junction. This raises the question as to whether this zone may functionally mediate not only the well-known absolute intrauterine fetal defense against maternal metastatic neoplasia, but also the defense of the fetus against maternal immunologic rejection.