With the advent of sophisticated automated blood processors, the collection of large numbers of granulocytes for transfusion has been made more practical in the past ten years. Harvesting granulocytes by filtration leukapheresis has been abandoned in most centers because of adverse reactions in both donors and recipients. Currently, both continuous and discontinuous flow centrifugation leukapheresis techniques are available. However, both corticosteroids and hydroxyethyl starch are required for optimal granulocyte collection. In this paper, we critically compare the three major cell separators used for the collection of granulocytes by centrifugation leukapheresis. All three instruments separate blood cells by centrifugation; the IBM-2997 and the Fenwal CS-3000 function by continuous-flow centrifugation, and the Haemonetics Model 30 by discontinuous-flow centrifugation. Factors such as the donor preapheresis white blood cell count, the blood flow rate through the machine and the centrifuge speed effect granulocyte collection. Comparisons will be made of the cost of the software for each machine, the time required for granulocyte collection, the convenience of set-up and tear-down, the amount of skill and experience demanded of the operator. Donor factors will be discussed. Since the blood processor may be used for other procedures including plateletpheresis, therapeutic plasma exchange, lymphocytapheresis and erthrocytapheresis, the pros and cons of each machine as used for some of these procedures will be included in the discussion. In our experience, total leukocytes and granulocytes (neutrophils) collected with the three different instruments varied only slightly when unstimulated donors were studied.(ABSTRACT TRUNCATED AT 250 WORDS)