Present knowledge of the alloantigenic status of the placenta, which makes it a natural semi-allogeneic allograft is briefly surveyed. The systemic immunoregulatory mechanisms operating during normal allopregnancy which are not a prerequisite for a successful pregnancy are recalled. The placenta--dependent local mechanisms, e.g. trophoblast dependent decidual suppressor cells, factor mediated and factor independent resistance to cell mediated lysis, are surveyed as well as some of the mechanisms of action of trophoblast regulatory factors, namely suppressor cell induction and inhibition of IL-2 dependent cell growth/activation. The main feature of the CBA x DBA/2 model of spontaneous abortions in mice, and its prevention by anti Balb/c leukocyte immunisation are described. It was shown that anti-T cell depletion prevents the anti-abortive effects of immunisation. Such a treatment is also able to restore normal placental weight in auto-immune MRL lpr mice, which are known to display excess seric CSF beta-like activity (CSFs being in vitro efficient growth factors for trophoblasts). Transfer of such cells into the MHC compatible CBA/J prevents resorbtions upon a subsequent mating with DBA/2. Thus, direct effects of CSFs beta 1 and beta 2 as anti abortifacient (IL3 and GM CSF) are described together with the abortificacient effects of TNFs and NKs activators.