In an attempt to optimise the dose of G-CSF for mobilisation of PBPC in allogeneic donors, four groups of six healthy male volunteers received lenograstim (glycosylated rHuG-CSF) at a dose of 3, 5, 7.5 or 10 micrograms/kg/day, respectively, for 6 days (days 1-6). All subjects underwent a 10 I leukapheresis. Lenograstim was well tolerated. Maximal mobilisation was observed on days 5 or 6, with a clear dose-response for all progenitor cell types (CD34+, CFU-GM, BFU-E, CFU-mix). The peak numbers of CD34+ cells/microlitre (mean, s.e.m.) were 30 +/- 5, 49 +/- 8, 44 +/- 5 and 122 +/- 30 in the 3, 5, 7.5 and 10 micrograms/kg groups, respectively. A good correlation was observed between the number of CD34+ cells in blood and leukapheresis product (LP), respectively. Increasing the dose of lenograstim did not increase the number of T cells in the LP. A comparison of LP and steady state BM CD34+ cells in paired samples from each individual, showed a higher proportion of primitive immunophenotypes (CDw90+, HLA-DR-, CD45RA-, CD33-) among LP CD34+ cells. We conclude that increased doses of G-CSF improve the mobilisation of PBPC, and that G-CSF favours mobilisation of primitive CD34+ cell subsets. Lenograstim 10 micrograms/kg/day for 6 days should provide a sufficiently effective mobilisation of PBPC in most healthy PBPC donors.