Although granulocyte transfusion therapy has been shown to be effective in infected granulocytopenic animals and humans, the relative effectiveness of granulocytes (PMN) harvested by continuous flow centrifugation (CFC) or by continuous flow filtration leukapheresis (FL) remains uncertain. Studies in vitro of morphology and granulocyte functions have suggested cells collected by FL may be damaged. To compare the function in vivo of granulocytes collected by different methods, dogs were made granulocytopenic with cyclophosphamide (CYT) and then transfused with granulocytes collected by CFC or FL. The local neutrophil mobilization (LNM) through a standard skin abrasion into a chamber containing a strong chemoattractant, autologous serum, was measured. Greater LNM was found after transfusions of CFC PMN than after transfusions of the same number of FL PMN (p less than 0.0003). This difference persisted even when the dose of FL PMNs was four times greater than that of CFC mn and when the FL donor was pretreated with steroids (p less than 0.001). These results suggest that during filtration leukapheresis, granulocytes are functionally altered and that their function in vivo may be compromised.