The fetuses of all outbred mammalian species present an immunogenetic set of paternal antigens that are foreign to the mother. Yet the fetus will survive the period of intrauterine gestation without rejection despite the fact that its mother will reject fetal or paternal tissues grafted at any other site. This apparent paradox of transplantation immunology still remains an enigma today even though it has been the focus of active research over the past twenty years. Medawar (1) hypothesized many years ago that the success of the fetus as an allograft could be explained as follows:(a) the conceptus was not immunogenic and therefore did not invoke a maternal immunologic response;(b) pregnancy altered the maternal immunologic response;(c) the uterus was an immunologically privileged site;(d) the placenta was an effective immunologic barrier; and (e) the fetus was not yet immunologiн cally competent. While many of these hypotheses did not withstand the test of time, they provided investigators with the opportunity to critically anaн lyze, and as a result enhance understanding of, this very complex phenome-