['The finding of reduced incidence of graft-versus-host disease (GVHD) associated with cord blood transplantation, compared to unrelated allogeneic bone marrow, could be related to the lower number of T cells infused in the cord blood (CB) inoculum or it might represent an intrinsic property of CB T cells. We investigated the in vivo function of human cord blood mononuclear cells (CBMC) after their adoptive transfer into lethally irradiated BALB/c radioprotected with severe combined immunodeficiency (SCID) mouse bone marrow.', 'The ability of human CBMC to engraft and produce antigen-specific alloreactive cytotoxic T lymphocytes (CTLs) and antibodies was determined by FACS, 51Chromium-release assay, and ELISA.', 'Recipients of human CBMC showed engraftment of high levels of both CD14+ and CD3+ cells. Human cells recovered from the peritoneal cavity of chimeric mice, 1 week after immunization with irradiated allogeneic cells, showed adult-level human CTL response against the immunizing cells, without further stimulation in vitro. In contrast, immunization with the tetanus (TT) antigen did not lead to the generation of anti-TT immunoglobulins (Ig) in the cord blood chimera. Furthermore, whereas the addition of purified adult T cells to the cord blood inoculum resulted in the enhancement of human Ig production of both IgG and IgM subclasses, it could not induce antigen-specific antibodies after immunization.'] Are t cells from newborn humans fully capable of developing into cytotoxic T lymphocyte effector cells in adoptive hosts?