The effects of both daily G-CSF administration and subsequent peripheral blood progenitor cell collection (PBPCC) by apheresis on 20 healthy adult donors were studied. All received daily G-CSF (filgrastim) 10 μg/kg for 5–7 days by subcutaneous injection. G-CSF administration was well tolerated, except for moderate bone pain and headache. Peak values of CD34+ cells were observed on days 5 (n=12) or 6 (n=8). In all donors a significant increase in CD3+, CD4+, CD8+, CD19+, and NK cells was observed on day 5 in relation to the baseline values. CD4/CD8 lymphocyte ratio was unmodified by G-CSF. None of the donors required a central venous line for PBPCC. Immediately after PBPCC, a platelet count below 100×109/l was observed in nine of 18 cases, although in all donors platelet counts were over 100×109/l 7 days later. A lymphocytopenia on day 7 following PBPCC was observed, although there was a tendency to achieve baseline values 30–90 days after the procedure. Mean numbers (±SD) of collected cells ×106/kg after a median of two (1–4) apheresis sessions and a median of 20 l (10–40) processed were: CD34+ 5.5 (±2.3), CD3+ 326 (±105), CD4+ 207 (±64), CD8+ 164 (±60), CD19+ 88 (±32), and NK cells 32 (±14). We conclude that G-CSF administration to healthy donors is a well-tolerated procedure which is associated with (a) obtaining a high number of hematopoietic progenitor cells, and (b) a significant increase in T, B, and NK cells in donors' blood. In addition, PBPCC by apheresis results in a moderate, rapidly reversible, and clinically irrelevant thrombocytopenia and a moderate lymphocytopenia, which tends to resolve within 3 months following the procedure.