Granylocyte bactericidal capacity, chemotaxis, hexose monophosphate shung activity (before and after phagocytic stimulus), and quantitative nitroblue tetrazolium reduction and enzyme content were examined in cells obtained by filtration leukaphresis (FL) and continuous-flow centrifugation (CFC). A decrease in the bactericidal efficiency of FL- produced cells compared to that of both normal and CFC-procured granulocytes was found; the decrease was 17% with a cell-to-bacteria ratio of 5:1, and 55% with a 1:1 ratio. Moreover, FL-acquired cells were often vacuolated and consistently contained less acid phosphatase and beta-glucuronidase than did normal granulocytes. When normal cells were incubated for 1–2 hr with nylon wool, 30% of the total acid phosphatase and beta-glucuronidase was released, with no evidence of cell death, thus suggesting degranulation. Similar results were obtained with glass, cotton, or polysulfone plastic fibers. Electron microscopic and peroxidase cytochemical studies of the adherence of normal granulocytes to nylon fibers were also carried out. After 30 min of incubation, cell-to-fiber attachment and cellular aggregation had occurred, although the cells per se appeared normal. After 60 and 120 min, other changes became apparent: (1) a decrease in the amount of cytoplasmic granules; (2) large, intracytoplasmic vaculoles; and (3) extracellular peroxidase on fiber surfaces. We conclude that granulocytes obtained by adherence to nylon fibers show both morphological and biochemical evidence of degranulation and diminished bactericidal capacity, and that these abnormalities may be causally related to decreased granulocyte survival in transfusion recipients.