We conducted a comparative study on a daily single versus a divided dose of G-CSF for G-CSF-induced mobilization of peripheral blood stem cells (PBSC) in eleven HLA-identical sibling donors of allogeneic PBSC transplantation (PBSCT). Six donors received double subcutaneous injections of G-CSF at a dose of 5 micrograms/kg x 2/day for 5 days (Group A), while the remaining five received single subcutaneous injection at a dose of 10 micrograms/kg/day for 5 days (Group B). The numbers of circulating CD34+ cells, myeloid progenitors (CFU-GM) and erythroid progenitors (BFU-E) reached peak values at day 5 of G-CSF administration in both groups. The mean number of CD34+ cells harvested per apheresis was 4.4 x 10(6)/kg (cells/body weight of each donor, range: 0.8-7.9 x 10(6)/kg) in Group A and 5.1 x 10(6)/kg (range: 3.0-9.0 x 10(6)/kg) in Group B. There were no significant differences between these two groups in total numbers of CFU-GM, BFU-E, or T-lymphocytes harvested. Adverse effects including mild to moderate bone pain and thrombocytopenia were transient and well tolerated. No difference was observed in the incidence of adverse effects between the two groups. These observations suggest that there is no difference in G-CSF-induced mobilization of PBSC between daily single and divided dose of G-CSF to collect a sufficient number of PBSC for engraftment after allo-PBSCT.