ABSTRACT: Concanavalin A (Con A) was used to study the suppressor function of human umbilical cord blood T cells. The suppressor activity and the phenotype of Con A-activated adult and cord T cells were compared using allogeneic adult mononuclear cell (MNC) response to pokeweed mitogen (PWM), phytohemagglutinin (PHA), and alloantigen as indicator culture systems. It was found that Con A-activated adult suppressor T cells were predominantly T8+cells and suppressed PWM-induced IgG production by adult lymphocytes, as well as PHA- and alloantigen-induced proliferation of allogeneic adult MNC. Con A-activated cord T cells, however, exhibited no significant suppression in any of the three indicator systems. Furthermore, using Con A-activated adult T8+cells as a source of suppression, the proliferative response of cord lymphocytes to PHA and alloantigen were as susceptible to suppression as those of adult lymphocytes. These results, together with previous findings that PWM-induced cord suppressor T cells were predominantly T4+cell-mediated suppression, suggest that fetus/newborn lymphocytes achieved their selective inhibition on adult lymphocytes through the T4+suppressor cell circuit.