ABSTRACT: The absolute numbers of B lymphocytes and of total and “active” T lymphocytes in peripheral venous blood (Mv) from 15 females at the time of normal term deliveries were found to be significantly less (p< 0.001) than in the fetal umbilical vein (Uv) or artery (Ua) or in the peripheral blood of 75 normal nonpregnant controls (Cv), suggesting that maternal cellular immunity at term is lowered. In 19 umbilical artery samples, titers of lymphocytotoxic antibodies (Cyt), expressed as the mean log of reciprocal titer values, were significantly higher (p< 0.01 in each case) than in matched maternal samples, against the following cell types: Maternal T cells (7.1 in Ua vs 1.21 in Mv sera); maternal B cells (3.23 vs 1.58); T cells (4.41 vs 1.38) but not B cells from other females at delivery; autologous T cells (2.9 vs 1.0); autologous B cells (1.88 vs 0.69); T (5.39 vs 0.81) and B (2.89 vs 1.25) cells from the paired Uv; T (3.78 vs 0.62) and B (2.64 vs 0.77) cells from the Uv of other newborn infants; and T (4.19 vs 2.0) but not B cells from controls (Cv). The highest Cyt titers in the umbilical artery samples were against maternal T lymphocytes. Immunofluorescence studies indicated that the Cyt antibodies were primarily IgG. Absorption of 13 other Ua sera with maternal T cells eliminated Cyt activity against both Mv and Cv T cells; absorption with Cv T cells eliminated the reaction against Cv T while reducing Cyt titers to Mv T lymphocytes. We conclude that the fetus produces lymphocytotoxic antibody specifically directed against maternal T lymphocytes, in addition to antibody against T lymphocytes of other adults.