The collection efficiency (CE) of the Fenwal CS3000 in collecting peripheral blood stem cells during post-chemotherapy recovery phase ranges from 58% to 73%. Recently filgrastim (recombinant methionyl human granulocyte colony-stimulating factor [G-CSF]) has also been shown to be effective as a mobilization agent although mobilization occurs during elevated and not low normal leukocyte counts. We compared the mononuclear cell (MNC) CE and the myeloid progenitor cell (CFU-GM) CE among 11 patients with G-CSF mobilization (33 procedures) and 19 patients during recovery following myelosuppression chemotherapy (93 procedures). Pre-apheresis leukocyte, neutrophil, MNC, and PB CFU-GM counts were significantly higher in the G-CSF group, while the granulocyte percentage in the apheresis products was similar in both groups. Both MNC CE (81.8 +/- 4.5% vs. 64 +/- 2.4%) and CFU-GM CE (79.5 +/- 10.5% vs. 55.8 +/- 3.5%) were higher in the G-CSF group. Only the pre-apheresis MNC count showed an independently significant correlation for both CE (P < .001). The higher CE in the G-CSF group can only be partly explained by a rise in MNC count during apheresis. These data suggest that the blood cell separator works better with leukocytosis, and especially with a higher MNC count. The improvement in CE is another benefit of G-CSF mobilization over chemotherapy mobilization.