We tested sera from 20 women [9 parous, 8 with no children, and 3 with abortion(s)] for inhibition in mixed lymphocyte culture (MLC). In these experiments, the responder (wife) lymphocytes were treated with autologous serum and rabbit complement and then tested for responses against stimulator cells from the husband and from third-party allogeneic donors. The results demonstrate that antibodies capable of inhibiting responses of wife's lymphocytes to husband's cells in MLC are present in sera from parous women, but not in women without children and in women with abortion(s). The MLC-inhibiting activity in parous women sera was in the IgG fraction. The results from immunofluorescence and absorption experiments suggest that the inhibition in MLC was due to antibodies directed against recognition sites on wife's T lymphocytes against husband's alloantigens. These observations suggest that antiidiotypic antibodies against husband-specific idiotypes on wife's lymphocytes could be induced by pregnancy and that maternal tolerance to fetus may be attributable to a similar mechanism occurring in vivo.