The effects of sera from pregnant women, human umbilical cords and nonpregnant healthy control individuals on human lymphocyte responses to phytohemagglutinin (PHA), pokeweed mitogen (PWM) and allogeneic lymphocytes (MLR) are described. Maternal sera invariably inhibited lymphocyte responses to PHA. In MLR and PWM-stimulated lymphocyte responses, however, some maternal sera had pronounced inhibitory effect while other maternal sera had none. Similar dissociation of inhibitory effects of lymphocyte responses to PHA and to PWM or in MLR was noted in some cord sera but not in the control sera we studied. The observations may reflect different serum factors impinging upon different lymphocyte subpopulations responsive to different mitogens. Preliminary attempts to relate the serum inhibitory effects to various serum components revealed that inhibition of MLR appeared to correlate with increases in alpha-2-globulins and beta-globulins. In contrast, inhibition of PHA responses was not similarly related to any one or two specific serum constituents.