Increasingly, allogeneic and even more often autologous bone marrow transplants are being done to correct a wide variety of diseases. In addition, autologous marrow transplants potentially provide an opportune means of delivering genes in transfected, engrafting stem cells. However, despite its widespread clinical use and promising gene therapy applications, relatively little is known about the mechanisms of engraftment in marrow transplant recipients. This is especially so in the nonablated recipient setting. Our data show that purified lineage negative rhodamine 123/Hoechst 33342 dull transplanted hematopoietic stem cells engraft into the marrow of nonablated syngeneic recipients. These cells have multilineage potential, and maintain a distinct subpopulation with "stem cell" characteristics. The data also suggests a spatial localization of stem cell "niches" to the endosteal surface, with all donor cells having a high spatial affinity to this area. However, the level of stem cell engraftment observed following a transplant of "stem cells" was significantly lower than that expected following a transplant of the same number of unseparated marrow cells from which the purified cells were derived, suggesting the existence of a "nonstem cell facilitator population," which is required in a nonablated syngeneic transplant setting.