We previously used peripheral newborn blood (NBB) as a possible in vivo experimental model for cord blood (CB) transplantation and showed that B10.D2 NBB cells successfully reconstituted adult (DBA/2 x B10.D2)F1 mice without causing graft-versus-host disease (GVHD), probably because of their phenotypic and functional immaturity. Here we investigated the influence of T-cell maturation occurring in NBB cells during the early postbirth period on the degree of engraftment, the incidence of GVHD, and the graft-versus-leukemia (GVL) potential. These parameters were compared in recipients grafted with bone marrow (BM) cells. We observed an increased percentage of CD4(+) mature T cells accompanied by the acquisition of proliferative responses to phytohemagglutinin (PHA) and to allogeneic cells of day-5 NBB cells. The capacity of day-2 NBB to engraft was moderately reduced and recipients developing GVHD were occasionally observed after the graft of day-5 NBB cells. No GVL effect was evidenced regardless of the time of postbirth blood collection. However, the GVL effect can be obtained by the delayed infusion of donor mature T cells to recipients grafted with day-0 NBB, without causing GVHD. In contrast, the same protocol applied to mice grafted with BM cells induced GVHD mortality of all recipients. Interleukin (IL)-10 but not IL-2 messenger RNA was expressed in NBB cells as opposed to BM cells. These findings suggest that, in terms of GVHD incidence, delayed infusion of mature T cells as post-transplant tumor immunotherapy would be more effective when applied after CB than after BM transplantation.