The mechanisms regulating long-term engraftment of primitive stem cells are largely unknown. Most conditioning strategies use myeloablative agents for experimental or clinical hematopoietic stem cell transplantation. Host conditioning regimens, in part, have been designed on the assumption that trans planted cells home to specific marrow sites and if these sites are occupied by host stem cells, engraftment will not take place. However, there is now evidence that stable long-term syngeneic engraftment may occur in the absence of host marrow stem cell depletion. To further study the association of engraftment with stem cell depletion, we investigated whether the marked egress of hematopoietic progenitor and stem cells from the marrow into the peripheral blood in C57BL6 mice following a single dose of cyclophosphamide (day 1) and four days of G-CSF (days 3-6) afforded an increased opportunity for long term syngeneic donor engraftment. During and after mobilization, glucose phosphate isomerase (GPT)-1(b) mice received 30 x 10(6) GPI-1(n) marrow cells without further myeloablation. The level of donor/recipient chimerism was assessed in cell lysates after six months. Increased long-term syngeneic donor engraftment was observed prior to mobilization (before day 6), during a period of active hematopoietic regeneration following the administration of cyclophosphamide. Hematopoietic regeneration was evidenced by a reduced but rapidly increasing marrow cellularity and an increased proportion of hematopoietic progenitors in S phase. In contrast, long-term syngeneic donor engraftment was not increased over controls during the period of maximum progenitor and stem cell mobilization (after day 5), At this time there were minimal numbers of progenitor and stem cells in the marrow. These data suggest that in the absence of host stem cell ablation, maximal engraftment does not occur during mar row progenitor or stem cell depletion, suggesting that the presence of "open" marrow sites is not a prerequisite for engraftment, The mechanisms for increased engraftment during progenitor cell regeneration following cyclophosphamide need further investigation. Understanding the mechanisms for engraftment without host stem cell ablation may allow strategies for improved long-term engraftment of syngeneic or autologous stem cells with reduced post transplant toxicity.