The essential information about materno-foetal immunologic interactions is reviewed. After a brief description of placental tissue (decidual cells from uterine mucosa, trophoblast), evidence that immunisation can be achieved by introduction of antigen into normal uterus is presented, although grafts performed in uterus of pregnant females are prolonged. Throphoblast cells bear specific antigens but histocompatibility antigens are usually not detectable at their surface. Proteins and cells from the foetus enter the maternal circulation and vice-versa, resulting in the production of antibodies against cell-associated foetal antigens by the mother. Some regulatory process of the maternal immune response have been investigated including blocking antibodies, maternal and foetal suppressor T cells. The biological roles of trophoblast and that of sustained microchimerism are underlined. Immunologic damage to the foetus can be produced only by inducing a cytotoxic cellular response before pregnancy. Possible analogies between immune responses to foetal allografts and tumors are discussed.