Mobilization of PBPC was investigated in 19 healthy matched sibling donors using two different rhG-CSF regimens. Five donors (median age 39 years, range 17 to 57 years) received 10 micrograms rhG-CSF/kg bw once daily subcutaneously (s.c.), while 14 donors (median age 34 years, range 19 to 56 years) were treated with 10-12 micrograms rhG-CSF/kg bw twice daily s.c.. Leukapheresis was started on day 4 of rhG-CSF administration. Cytokine priming as well as collection of PBPCs were well tolerated. Application of twice daily rhG-CSF resulted in a higher yield of CD34+ cells in leukapheresis products than injection of once daily rhG-CSF. This high-dose twice daily rhG-CSF regimen is well tolerated and results in reliably high numbers of progenitor cells in the leukapheresis product in healthy donors, therefore collection as well as subsequent selection has been facilitated.