The functional maturity of human umbilical cord blood B lymphocytes and the immunoregulatory activity of cord T lymphocytes were assessed by measuring the in vitro immunoglobulin production by B cells from either cord or adult blood. Supernatants from 48-hr pokeweed-mitogen (PWM) stimulated cord or adult lymphocyte cultures were added to cord or adult B cell cultures in the presence of PWM; a significant amount of immunoglobulin was produced in adult B cell cultures only. Adult B or T cells were then cocultured with cord T or B cells; a significant amount of immunoglobulin was again found only in adult B cell cultures. These results indicated that cord B cells were functionally immature and that cord helper T cell function was adequate but masked by excessive suppressor activity. Indeed, addition of cord T cells but not of allogeneic adult T cells to PWM stimulated adult lymphocyte cultures inhibited their immunoglobulin production; this confirmed cord T cells' increased suppressor activity. Cord T cells were not intrinsically suppressive since they failed to suppress immunoglobulin production by Epstein-Barr Virus (EBV) transformed B cells. They could be activated, however, by PWM or allogeneic cells (in mixed lymphocyte cultures) and their effect was mediated via soluble factor(s) as demonstrated by the suppressor effect of these culture supernatants on immunoglobulin production by unfractionated adult lymphocytes. In contrast, when these supernatants were added to T cell-depleted adult lymphocyte cultures, enhancement rather than suppression was observed. These results indicated that the soluble factor(s) released by Cord T lymphocytes was not suppressing per se but induced suppression through activation of suppressor cells.