Peripheral blood stem cells (PBSC) are a good source for bone marrow reconstitution after intensive chemotherapy. The ability to transplant PBSC between allogeneic subjects would be a key step forward in the application of this procedure. For this purpose, we examined the mobilization and recovery of PBSC in healthy volunteers who were given recombinant human granulocyte colony-stimulating factor (G-CSF). Three informed healthy volunteers were injected with G-CSF subcutaneously at a dose of 2.5 micrograms/kg for 6 successive days and at 5.0 micrograms/kg for the following 4 days. The concentration of PBSC was assessed daily by CFU-GM and BFU-E assays, both of which peaked on the sixth to seventh day of G-CSF administration. Comparison between colony assays and hematological parameters revealed that flow cytometry analysis of CD34+ cells by mononuclear cell gating is a rapid and convenient method to quantify mobilized stem cells. The maximum numbers of CFU-GM were 432, 665, and 1386 colonies/ml blood. It is calculated that sufficient amounts of stem cells for transplantation (at least 1.0 x 10(5) CFU-GM/kg) could be obtained by leukapheresis of 5 to 201 blood when the peak was attained. This trial confirmed the feasibility of allogeneic transplantation using PBSC from healthy volunteers who have received G-CSF.