The idea that immunosuppressor cells in the embryo may play a role in immunosurveillance, by preventing a response of emerging embryonic potentially autoreactive cells and possibly also invading maternal cells has attracted considerable attention. Indeed, in the past few years experimental evidence has accumulated showing suppressor cell function in the embryo and early after birth [see ref. 1 for review]. Furthermore, it has been indicated that fetal cells can interfere with the response of maternal lymphocytes [2, 3]. The present communication represents a brief review of our studies, we have concentrated on identifying and characterizing the embryonic immunosuppressor cells in this respect.