We investigated a dose-escalation effect of G-CSF (5, 10, and 15 micrograms/kg) on mobilization of committed and primitive hemopoietic progenitor cells, including CFU-GM, BFU-E, and long-term culture-initiating cells (LTC-IC) in addition to CD34+ cells and yields of progenitor cells in PBSC harvests obtained by leukapheresis of healthy adult donors. Results indicate that the mobilization of these progenitor cells is both dose and time dependent. Despite the very small number of healthy donors studied, it is estimated from our data that a sufficient number of CD34+ cells for allogeneic PBSC transplant (PBSCT) could be collected using a 5 day administration of 10 micrograms/kg of G-CSF to normal adult donors. Adverse effects include general fatigue and bone pain in most of the donors and fever and headache in some. These symptoms were well tolerated in most instances. Laboratory test abnormalities, including transient thrombocytopenia, increased platelet aggregation, and increased serum levels of some liver enzymes, were induced by G-CSF administration, but all were reversible within a short time. These observations suggest that hemopoietic stem cells for allogeneic PBSCT can be mobilized by short-term administration of a relatively high-dose G-CSF.