G-CSF administered to healthy volunteers at a dose of 3 microg/kg for 5 days mobilized colony-forming units granulocyte-macrophage (CFU-GM), burst-forming units-erythroid (BFU-E), and long-term culture-initiating cells (LTC-IC) to a maximal level on day 4 or 5. To determine the number of primitive hematopoietic progenitors in the peripheral blood, mononuclear cells (MNCs) or CD34+ cells were cultured in a long-term culture (LTC) system. We defined the colonies produced by cells during LTC at week 2 as differentiated progenitors and those at week 5 as primitive progenitors. G-CSF administered to healthy volunteers increased the number of differentiated progenitors from day 4 and primitive progenitors on days 3-5. Enriched CD34+ cells from healthy volunteers treated with G-CSF (PB-G) or without it (PB-SS), and patients treated with chemotherapy plus G-CSF (PB-CG) were subjected to LTC for 7 weeks. PB CD34+ cells from PB-G contained less primitive progenitors than those from PB-CG. Therefore, in healthy donors administered G-CSF at a dose of 3 microg/kg, the number of PB CD34+ cells should be harvested as much as possible to perform allogeneic peripheral blood stem cell transplantation (PBSCT).