Despite the wide use of G-CSF for mobilization of PBPC the best dose and schedule of G-CSF has not been definitively established. In this study we have compared three different schedules of G-CSF for mobilization of PBPC in normal donors including a single daily dose of 10 microg/kg/day for 5 days (21 donors) and doses of 6 (21 donors) or 8 microg/kg/12 h (6 donors) for 5 days. We demonstrate that G-CSF at doses of 6 and 8 microg/kg/12 h mobilizes significantly more CD34+ cells/ml of blood (83.3 +/- 6.7 and 121 +/- 6.9, respectively) than 10 microg/kg/day (71.6 +/- 6.5). Mobilization with 6 or 8 microg/kg/12 h of G-CSF was also associated with collection of significantly more CD34+ cells in comparison with 10 microg/kg/24 h (2.24 +/- 1.2 and 2.46 +/- 1.22 vs 1.15 +/- 0.8 CD34+ cells/kg of donor/blood volume). PBPC collection was associated with a significant decrease in platelet count which was not significantly different between the three groups. Ten days after the last PBPC collection platelet counts were within normal limits while there was a decrease in WBC and ANC. We conclude that G-CSF administered every 12 h at doses of 6 microg/kg provides better CD34+ cell yield than 10 microg/kg once a day in normal donors which may translate into a decrease in the number of aphereses required to obtain enough numbers of CD34+ cells for allogeneic PBPC transplant.