Objective immunological methods of determining the mother's immunological response to her husband's and cord leukocyte antigens are being used in a three-year study of women who develop severe preeclampsia during pregnancy. The maternal leukocyte response to the nonspecific mitogen phytohemagglutinin and nonspecific mitogen phytohemagglutinin and to paternal and cord leukocyte antigens in the mixed leukocyte culture suggests a tendency for toxemic women to be relatively hyporesponsive. No effect of autologous maternal serum is demonstrated in the group as a whole. Women's cord leukocyte response to their own maternal antigens is less in toxemic pregnancies. A significant immunosuppressive effect of autologous cord serum is observed in preeclamptic pregnancies. Serum seromucoid levels are elevated in the toxemic women during and after the index pregnancy; also, toxemic women show a lack of anti-HL-A antibody formation. Such data suggest, but do not prove, that immune mechanisms may be operative in preeclampsia.