The earliest investigations into the properties of stored blood for purposes of transfusion clearly revealed the necessity for careful filtration of such material before use. That fibrin particles, clumps of white blood cells or disintegrated platelets, and even small blood clots were to be expected in blood stored over a relatively few hours was quickly apparent.1However, a few deaths with multiple pulmonary emboli from this material were reported before the extreme importance of adequate filtration of stored blood and plasma was fully appreciated. A common method of filtration in the early days was that of simply pouring the blood through several layers of ordinary sterile gauze.This method, of course, was clumsy, open to contamination and inadequate in removing small particulate matter. Furthermore, considerable cotton lint was always added to the filtrate. Undoubtedly, many of the so-called pyrogenic reactions to transfusions in the early days of blood storage