Human chorionic gonadotropin (hCG) in physiological retroplacental concentration has been shown to possess the capacity of inducing human lymphocytes which are subsequently competent to depress an antibody response induced by purified protein derivative of tuberculin, phytohemagglutinin, lipopolysaccharide and pokeweed mitogen. These suppressor cells inhibited the T cell-dependent mitogen-induced activation of lymphocytes synthesizing IgM, IgG and IgA. No suppression by hCG-induced cells was observed in Epstein-Barr virus activated cell cultures, indicating a T cell origin of the target cell. It is suggested that this may represent a mechanism for the cellular basis of an hCG-induced immunosuppressive effect in pregnancy.