An immunological response from mother to foetus is known to exist during pregnancy. Multiple mechanism ensure that this response is not one of rejection but of "tolerance" of the foetus, and the placenta, as the site of maternal-foetal exchanges, plays a prominent role in this respect: (1) the placenta cell bears foetal antigens, half of which are of paternal origin; it constitutes therefore the antigenic stimulus of maternal immune response; (2) this response can be modulated quantitatively through placental hormones and glycoproteins - an action which has been demonstrated in vitro but which may well take place at utero-placental level; (3) finally, the placenta can act qualitatively on the maternal response itself by reducing cytotoxic reactions responsible for graft rejection and by stimulating immunosuppressor cell populations. The therapeutic applications of some of these properties of the placenta are being investigated in the fields of rheumatology and renal transplantation. They will be considered in obstetrics only when more precise data are available concerning the immunopathological mechanisms involved in spontaneous abortion, preeclampsia and foetal growth retardation.