The use of peripheral blood stem cells instead of bone marrow as the source of haemopoietic cells for allogeneic transplantation is being increasingly explored. We have analysed data from 17 normal donors who underwent stem cell mobilization for allogeneic transplantation with an identical protocol using G-CSF at a dose of 10 micrograms/kg/d, with the first leukapheresis (LP) on the day following the fourth dose of G-CSF. Both G-CSF administration and leukapheresis were well tolerated. Donors under-went a median of two leukaphereses (range one to three) and a median of 6.80 x 10(6) CD34+ cells/kg recipient weight (range 2.4-15.6 x 10(6)) were collected. The median number of CD34+ cells per kg donor weight was 6.05 x 10(6), when corrected for a 12 litre leukapheresis, this gave a median total of 3.89 x 10(6) CD34+ cells/kg donor weight. When analysed with respect to factors which might influence the efficacy of mobilization, male donors were associated with a superior yield. The median number of CD34+ cells/kg/LP harvested was 4.96 x 10(6) in males and 2.79 x 10(6) in females (P < 0.05). The results suggested that, given a recipient of 75 kg, in a male donor a single 12 litre leukapheresis should yield sufficient CD34+ cells (4 x 10(6)/ kg), whereas a female donor would be likely to need two leukaphereses. Age was not found to affect donor yield. In summary, these data confirm that leukapheresis is a safe procedure in normal donors and suggest that males may be more efficient mobilizers of stem cells than females.