Primed peripheral blood hematopoietic stem cells (PBSC) generate and sustain lymphohematopoiesis in myeloablated animals, and recent reports indicate that allogeneic transplantation using PBSC grafts may be feasible in humans. A major concern with the use of PBSC transplants is that permanent engraftment may be limited because of lack of sufficient numbers of primitive progenitor cells in the graft. In the present study, in vitro colony formation and immunophenotype of CD34+ cells in PB of healthy adults during short-term granulocyte colony-stimulating factor (G-CSF) administration were compared with that of CD34+ cells in normal bone marrow (BM). The number of CD34+ cells mobilized to PB peaked at day 4 or 5 of G-CSF administration. The phenotypic profile of CD34+ PB cells showed a substantial increase in the percentage of CD34+CD13+ and CD34+CD33+ cells (myeloid progenitors) and a corresponding decrease in the percentage of CD34+CD10+ and CD34+CD19+ cells (B lymphoid progenitors) compared with CD34+ BM cells. The other subsets studied, including CD34+CD38- and CD34+HLA-DR- cells, were present in both compartments in similar proportions. Furthermore, primed CD34+ PB cells were enriched for colony-forming cells (CFC) and displayed an increased clonogenicity when compared with their counterparts in BM. A comparison between a postulated PBSC graft and an average BM graft is presented, showing that such PBSC grafts will be enriched for CD34+ cells as a whole, CD34+CD33+ cells, and colony- forming cells (CFC), factors which have been shown to correlate to acceleration of hematologic reconstitution and reduction in requirements for supportive care in autografting. Hence, we predict that allogeneic transplantation using G-CSF-primed PBSC grafts will result in a more rapid hematologic reconstitution after myeloablative conditioning than BM grafting. The question of whether PBSC allografting will result in permanent engraftment and clinical benefits as observed in autografting has to be determined in prospective clinical studies.