ABSTRACT:Primiparous and multiparous healthy pregnant women were tested at the end of the gestation period, or immediately after delivery, for their lymphocyte reactivity to paternal or neonatal cells in mixed lymphocyte culture and in cell‐mediate cytotoxicity assays. Freshly isolated maternal lymphocytes had no spontaneous cytotoxic activity against PHA‐activated neonatal or paternal lymphocytes. In conventional 6‐day mixed lymphocyte cultures and in cytotoxicity assays, maternal lymphocytes displayed a response similar to that of paternal or third party lymphocytes when stimulated with neonatal, paternal, or third party lymphocytes. By contrast, in 3‐day mixed lymphocyte cultures maternal cells had a selectively lower response to paternal antigens (expressed either on cord blood cells or on paternal lymphocytes), than to unrelated alloantigens. Removal of T‐lymphocytes with IgG receptors (T), or of T‐lymphocytes reacting with OKT8monoclonal antibody, corrected the depressed response of maternal cells in these 3‐day cultures. It is suggested that circulating maternal lymphocytes contain antigenspecific suppressor cells characterized by their membrane receptors for IgG and