One of the most intriguing shortcomings of modern immunology, especially transplantation immunology, is its failure to provide a satisfactory final explanation for the consistent non-rejection of immunogenetically alien conceptuses in utero, even in specifically preimmunized females. Certainly, there is no shortage of hypotheses. Over the past few years various observations have utterly refuted the simplistic notion that the much-mated but nulliparous or gravid females are immunologically unaware of the presence and activities of allogeneic cells transiently or chronically within their reproductive tracts. This evidence has engendered the belief that some kind(s) of active response(s) on the part of the female, following early recognitive events, plays an important, if not essential, role, in conjunction with adaptive hypoantigenicity of the trophoblast, not only in conferring selective benefits upon the conceptus from the time of implantation, but also in affording it protection from the possible hazards of a cellular immunity. Evidence is emerging that during mating, implantation, placentation, and gestation (1) various fetal and histocompabibility antigens are presented to the mother in a unique manner; (2) she does respond to these; and (3) her responses aid in the establishment and maintenance of a harmonious state of immunological coexistence with her fetus. There are reasonable grounds for believing that a complete understanding of the immunobiology of the maternal-fetal relationship may facilitate significant advances in both transplantation and tumour immunology.