Febrile transfusion reactions are encountered occasionally in patients receiving platelet concentrates that contain contaminating leukocytes. To remove heavier cellular elements, a 400-ml platelet pooling bg with tapered sides and a pouch at the base was designed to trap these cells after centrifugation. Two versions of this bag were tested: the initial prototype with a 3-ml pouch and a modified version with a 4-ml pouch. Platelet concentrates were pooled in batches of 6 per bag and, after sampling, were centrifuged between 186 and 600 x g for 7 to 10 minutes. The pouch on each bag was then clamped off, and samples from the primary bag were withdrawn so tht the percentage of cellular elements remaining could be determined. At optimal centrifugation conditions (390 x g for 10 minutes) with the initial prototype of the bag, average cellular decreases were: leukocytes, 75.9 percent; red cells, 84.1 percent; and platelets, 9.8 percent. Twenty-two components prepared in this manner were infused into 12 patients with histories of febrile transfusion reactions. Febrile reactions were markedly reduced or absent in ten patients, a strong febrile reaction due to incorrect component preparation occurred in one, and a marked allergic reaction occurred in one. The 4-ml pouch gave much better production results with greater than 95 percent leukocyte removal and less than 5 percent platelet loss. This system removes sufficient leukocytes from platelet concentrates that the risk of febrile transfusion reactions is reduced significantly.