The fetus resulting from an allogeneic (interstrain) mating represents a type of graft that is not rejected by the mother. Nevertheless, the maternal immune system seems to recognize and to react to the presence of the fetus in a number of ways. One such manifestation is significant enlargement of the lymph nodes that drain the uterus (DLN) of pregnant rodents. We have tested the DLN lymphocytes of mice for reactivity to paternal H-2 alloantigens after interstrain mating. The DLN lymphoid cells obtained from pregnant mice killed fewer newborn F1 recipients in a graft-vs-host mortality assay, and generated less cytotoxic T cell activity against paternal H-2 antigens both in vivo and in vitro. In vitro mixing experiments demonstrated the presence of a cell-associated suppressor activity in the DLN of pregnant mice. This suppressor proved resistant to treatment with mitomycin C, and appeared in the DLN early in pregnancy.