Retroplacental blood lymphocytes (RPL) in human pregnancy were studied for proliferative response to related fetal lymphocytes and it was compared with that of peripheral blood lymphocytes (PBL) of the same donor. RPL proliferated poorly to related cord blood lymphocytes in 9 of 10 cases when compared with autologous PBL. RPL also showed lower proliferative response to unrelated adult lymphocytes in 7 of 12 cases. Proliferation of RPL was weaker to related cord blood lymphocytes than to allogeneic adult lymphocytes. These results indicate that the response of T cells at feto-maternal interface is impaired, especially to fetal antigens.