In a search for the optimal method to harvest peripheral blood progenitor cells (PBPC) without myeloablative chemotherapy, we administered recombinant human granulocyte colony-stimulating factor (G-CSF) to adult, healthy volunteers and investigated the mobilization rate of the PBPC. The consecutive subcutaneous administration of G-CSF in a dose of 2 micrograms/kg/d for 5 days significantly increased colony-forming units-granulocyte/macrophage (CFU-GM) up to 2340+/-980 per mL whole blood with a 30+/-21-fold mobilization of PBPC, range 11-to 76-fold. Burst-forming units-erythroid (BFU-E) and mixed erythroid progenitors (CFU-Mix) were also increased, and the mobilization rate was 8.4+/-4.5-fold and 7.6+/-4.7-fold, respectively. A time course study of PBPC after final G-CSF injection was done for 0 to 30 hours on days 1, 3, and 5. After a single administration of G-CSF (day 1), there was no increment of PBPC during the subsequent 30 hours, but the white blood cell count (WBC) markedly increased. Three days after the injection of G-CSF, no increase of CFU-GM was seen during the first 2 hours, then a significant, time-dependent increase occurred for up to 30 hours. Five days after the injection of G-CSF, there was no increase of CFU-GM during the first 2 hours, and the significant increase at 24 hours was maintained for up to 30 hours. BFU-E and CFU-Mix showed the same pattern of expansion as seen with CFU-GM. Thus, subcutaneous administration of 2 micrograms/kg/d G-CSF for 5 days will lead to a successful PBPC harvest for transplantation, and the most suitable period for this is 24 to 30 hours after the final G-CSF administration.