Despite the wide use of G-CSF for mobilization of PBPC the best dose and schedule of G-CSF has not been definitively established. Zn 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 mu g/kg/day for 5 days (21 donors) and doses of 6 (21 donors) or 8 mu g/kg/12 h (6 donors) for 5 days, We demonstrate that G-CSF at doses of 6 and 8 mu g/kg/12 h mobilizes significantly more CD34(+) cells/ml of blood (83.3 +/- 6.7 and 121 +/- 6.9, respectively) than 10 mu g/kg/day (71.6 +/- 6.5), Mobilization with 6 or 8 mu g/kg/12 h of G-CSF was also associated with collection of significantly more CD34(+) cells in comparison,vith 10 mu g/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 mu g/kg provides better CD34(+) cell yield than 10 mu g/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.