Allogeneic bone marrow transplantation (BMT) is a potentially curative treatment for both inherited and acquired diseases of the hematopoietic compartment; however, its wider use is limited by the frequent and severe outcome of graft-versus-host disease (GVHD). Unfortunately, efforts to reduce GVHD by removing donor T cells have resulted in poor engraftment and elevated disease recurrence. Alternative cell populations capable of supporting allogeneic hematopoietic stem/progenitor cell engraftment without inducing GVHD could increase numbers of potential recipients while broadening the pool of acceptable donors. Although unfractionated CD4⁺ T cells have not been shown to be an efficient facilitating population, CD4⁺CD25⁺ regulatory cells (T-reg's) were examined for their capacity to support allogeneic hematopoietic engraftment. In a murine fully major histocompatibility complex (MHC)-mismatched BMT model, cotransplantation of donor B6 T-reg's into sublethally conditioned BALB/c recipients supported significantly greater lineage-committed and multipotential donor progenitors in recipient spleens 1 week after transplantation and significantly increased long-term multilineage donor chimerism. Donor engraftment occurred without GVHD-related weight loss or lethality and was associated with tolerance to donor and host antigens by in vitro and in vivo analyses. Donor CD4⁺CD25⁺ T cells may therefore represent a potential alternative to unfractionated T cells for promotion of allogeneic engraftment in clinical hematopoietic cell transplantation. (Blood. 2005;105:1828-1836)