To investigate whether granulocyte colony-stimulating factor (G-CSF) administration to donors before harvest may lighten the burden imposed on them and accelerate the bone marrow (BM) recovery, we administered 2 microgram/kg/d of G-CSF for five consecutive days before the marrow harvest. All of the donors tolerated the G-CSF administration well without severe adverse events. After 5 d of G-CSF treatment, CD34+ cells and granulocyte-macrophage colony-forming units (GM-CFU) in the donors' BM exceeded baseline values by 4.2-fold (range 0.71-316) and 1.6-fold (0.28-118) respectively. The concentration of total nucleated cells (x 107/ml) in the graft increased from 1.61 (0.95-3.23) to 2.44 (1.27-4.01). Although we collected 1020 ml of BM and obtained 1.50 x 1010 nucleated cells from unprimed donors, 940 ml of BM were sufficient to obtain 2.14 x 1010 nucleated cells from primed donors. However, G-CSF-primed BM did not shorten the time to tri-lineage engraftment and the duration of hospitalization compared with unprimed BM, although primed BM contained more CD34+ cells than baseline values. We consider that the advantages of BM priming are not the acceleration of BM recovery but rather the reduction of blood loss during BM harvesting.