Febrile transfusion reactions due to leukoagglutinins are commonly seen in multitransfused patients. It has been suggested that reduction of the number of leukocytes per transfusion to 0.5 x 10(9) or less would prevent nonhemolytic febrile transfusion reactions in the majority of patients. Therefore, we have compared the ability of several filters to reduce the leukocyte content of stored whole blood drawn into citrate-phosphate-dextrose. The leukocyte absorption filters, Imugard IG500 and Erypur, produced 98.3 +/- 2.4 and 98.3 +/- 1.7 percent leukocyte depletion, respectively. The leukocyte adherence filter, Leuko-Pak, produced a depletion of 54.6 +/- 14.7 percent. The microaggregate filters, Biotest MF10B Microfiltration set, Ultipor Blood Transfusion Filter SQ40S, and Travenol 20 micron High Capacity Transfusion Filter, produced 37.5 +/- 10.8, 13.3 +/- 11.2, and 44.5 +/- 11.3 percent leukocyte depletion, respectively. It would appear that of the filters tested, the Imugard IG500 and the Erypur are the only filters which can invariably reduce the absolute number of leukocytes in a unit of stored whole blood to below 0.5 x 10(9).