Ontogenic development of suppressor cells was studied in mouse embryos. It was found that liver cells of embryos at various stages of gestation were capable of interfering with the mixed leukocyte culture (MLC) reaction. The MLC reaction was also inhibited by embryonic spleen and thymus cells. However, the latter were inhibitory only when originating in 16− and 17-day-old embryos. Embryonic liver cells were also found to interfere with the graft-versus-host (GVH) response induced in neonatal (BALB/c × C57BL)F, or (C3H × C57BL)F1 mice by adult parental-type C57BL spleen cells. The effect was expressed in the mortality rates but was less pronounced in the splenomegaly reaction measured in vivo or in an in vitro test system. Suppression of the GVH mortality response was manifested predominantly when the liver cells were syngeneic with the effector cells. In contrast, the MLC reaction was inhibited by liver cells syngeneic or allogeneic with the effector cells. The possible developmental patterns of these cells are discussed.