ABSTRACT: The fetus resulting from allogeneic mating in an outbred population such as man represents an antigenic graft against which the mother mounts an immune response. However, the type of immunity elicited by the “fetal allograft” does not appear to mediate graft rejection. This observation suggests that suppression of those types of immune responses that cause graft rejection may account for fetal survival. Allografts placed experimentally within the uterus appear to enjoy prolonged survival compared to grafts at other sites, and this latter finding suggests that localized suppression of graft rejection may exist within the uterus at the maternal‐fetal interface. Previous studies have shown that suppressor cells which develop in the lymph nodes draining the uterus (DLN, paraaortic and renal lymph nodes draining the uterus) of allogeneically mated C3H and CBA strain mice are small lymphoid cells that sediment at 3 mm/h in unit gravity; arise early during first pregnancy, reaching maximum activity near the time of implantation (“grafting”); selectively inhibit the generation of cytotoxic T cells (cytotoxic T lymphocyte—CTL—or killer T cells) in vitro and in vivo; and are absent in the DLN of CBA mice which spontaneously resorb their litters. Furthermore, this suppressor cell population appears to be concentrated within the uterine decidua, and both DLN and decidual suppressor cells are under hormonal control. We now report that initial onset of suppression in decidua occurs more rapidly than in DLN after mating. The suppressor cell population in both DLN and decidua is resistant to destruction by anti‐T serum plus complement and anti‐Ly sera plus complement, and preliminary studies show that suppressor cell activity is associated with a population of granulated small lymphocytes. A soluble suppressor activity can be obtained from the decidual lymphoid population similar to that obtained in studies of the DLN suppressor cell. Small numbers of suppressor cells can prevent the infiltration of sponge matrix allografts by cytotoxic T cells in vivo. Thus, a non‐T suppressor cell population may accumulate within the uterus and protect the fetal allograft from cell‐mediated immune rejection.