ABSTRACT: This investigation involves 3 women with habitual abortion and 2 women with multiple normal pregnancies. Extensive clinical investigations failed to reveal any cause for the abortions. All husbands were ABO-compatible with their respective wives. The women shared three, two, and zero of five identified HLA-A antigens with their respective husbands. None of the women had any cytotoxic antibody. Lymphocytes from all three women reacted normally in one-way mixed lymphocyte cultures to their husbands' alloantigens. Cell-mediated lympholysis was similar in couples with habitual abortion and in those with repeated normal pregnancies. The women's lymphocytes' proliferation was significantly suppressed by lymphocytes from their husbands and by those from most male controls in two-way mixed lymphocyte culture and double chamber experiments. Two women with 6 and 5 live-born children, respectively, served as controls. Their lymphocytes were not suppressed by lymphocytes from their respective husbands or by those from most male controls in two—way mixed lymphocyte cultures. Their reactions in one—way mixed lymphocyte cultures and in cell—mediated lympholysis, however, did not differ from those of the women with habitual abortion.