||Bone marrow transplantation has revolutionized human medicine by extending the life span, and even curing, individuals afflicted with cancer and other pathological hematologic conditions. However, the mechanisms that mediate the engraftment of hematopoietic stem cells (HSCs) after transplantation remain largely obscure. A more comprehensive understanding of the factors that positively and negatively effect engraftment of stem cell and progenitor cell populations is needed to achieve greater success in performing bone marrow and stem cell transplants by preventing the occasional transplant failure. Evaluating the ability of several defined stem cell and progenitor cell populations to contribute to WBC recovery following transplantation into irradiated mice, we found the Rholow subset of stem cells to be solely responsible for bone marrow reconstitution at long periods of time posttransplant. In addition, we identified a lymphoid progenitor cell population that possessed a phenotype very similar to stem cells cells (Sca-1+ c-kit+ Lineageneg Thyneg) and that mediated lymphoid reconstitution of the bone marrow and blood at very early times posttransplant. The results of these studies also demonstrated that the use of different selection protocols for isolating the Thyneg population could drastically diminish the early reconstitutive properties of these cells. Specifically, the c-kit allophycocyanin conjugate derived from the 2B8 clone, but not the c-kit biotin conjugate derived from the ACK4 clone, suppressed the early lymphoid recovery demonstrated by this cell population. Lastly, we devised a new murine model for performing bone marrow transplants that involved transplanting two bone marrow grafts separated by a time interval of several days. Subsequent serial transplantation of bone marrow from reconstituted recipients resulted in significantly greater peripheral blood reconstitution by the second graft in tertiary recipients. However, this was only found to be true when the second graft was administered to primary recipients 3 or 5 days after the first graft. We subsequently revealed there was a 4-fold increase in HSC numbers in primary recipients that received the second graft after a 5-day delay. This supports the idea that HSCs may lose their self-renewal and proliferative properties following transplantation. Therefore, this modified transplant regime may prove useful in preventing graft failure after very long periods of time posttransplant.