Allogeneic bone marrow transplantation currently plays a critical role in the treatment of leukemias and inherited disorders of hematopoiesis, and it shows great promise for the treatment of numerous other diseases. The problems of graft-vs-host disease (GVHD) and failure to engraft, however, remain formidable obstacles to the widespread use of this therapy. Successful transplantation of purified populations of hematopoietic stem cells (HSCs) can theoretically avoid the problem of GVHD, since purified HSCs lack the mature elements that allow the graft to mount a response against the host. In previous studies from our laboratory, a population of purified HSCs (Thy-1loLin-/loSca-1+) was isolated from mouse bone marrow (BM). These cells represent approximately 0.05% of BM cells and are capable of self-renewal and long-term reconstitution of all blood lineages. Here we report long-term engraftment of these purified HSCs transplanted in mice across successively more difficult allogeneic-histocompatibility barriers. Transplantation of purified HSCs were quantitatively compared with whole bone marrow (WBM) grafts containing equivalent numbers of stem cells. The mouse strain combinations tested were parent transplanted into F1 (Hh disparate), minor histocompatibility complex (mHC), and major histocompatibility complex (MHC) plus mHC disparities. One of the recipient strains studied for MHC-disparate transplantations was that of spontaneously autoimmune diabetic mice. Recipient mice were administered lethal doses of whole-body irradiation in the presence or absence of antibodies directed against natural killer (NK) cell-associated determinants and/or monoclonal antibodies against the CD4+ T cell subset. We find that as the barrier to transplantation increases, greater numbers of HSCs are required for radioprotection and engraftment. In all cases, stable hematopoietic chimeras were generated with HSCs alone, but 10-60 times the number of HSCs was required for radioprotection of mice transplanted across allogeneic or semiallogeneic disparities as compared to Ly-5 congenic differences. Furthermore, we demonstrate a clear advantage of WBM vs HSCs with regard to tha ability to engraft [corrected]. Chimeric mice showed no symptoms of GVHD, and their T cells were unable to induce GVHD in neonatal mice expressing H-2 antigens of donor and host. These data confirm that a cell population resident in WBM and distinct from purified stem cells is important in facilitating hematopoietic engraftment, in this case, of purified allogeneic HSCs. The differences in engraftment between WBM and HSCs could be reduced significantly by the addition of antibodies directed against NK determinants to the host preparative regimen. Similarly, since antibodies directed against host NK-associated antigens can reduce the barrier to allogeneic HSC engraftment, an interaction between the facilitating population within donated WBM and a resistant host population with NK determinants is implied.