Human chorionic gonadotropin (HCG) has previously been considered to be immunosuppressive and, thus, by prohibiting maternal rejection of the fetal transplant, to be one of the factors responsible for the successful outcome of pregnancy. The mechanisms by which HCG exerts its pregnancy retaining effect is, however, as yet unknown. The present study shows that HCG has the capacity in mice of inducing lymphocytes which are subsequently competent to depress a polyclonal antibody response induced by different B cell mitogens. It is therefore suggested that HCG may exert its feto-protective action by inducing suppressor T cells. These lymphocytes would thus prevent the activation of cells which are responsible for transplant rejection.