On the basis of results of our research and review of literature, the complex of immuno logical influences, operating during the development of the human fetus, were evaluated. It is obvious that during the early stages of pregnancy the conceptus is protected by non-specific mechanisms, i.e. hormonally (HCG, progesterone) and by certain properties of the trophoblast (barrier function, immunologically inert surface). Specific immunological tolerance is formed by gradual penetration of trophoblast particles and later by penetration of fetal blood cells into maternal circulation. Thus a specific suppression of maternal T lymphocytes against fetal antigens develops, other immunological functions being intact. - Following a strong antigenic stimulus (e.g. Rh-D), isoimmunization of the mother and serious risk for the fetus occur. Immunological causes of abortion could not be unequivocally proved in recurrent abortions. The explanation of the origin of EPH-gestosis on the basis of toxic action of immunocomplexes is highly probable, however the laboratory and experimental proof is still lacking.