Aggregates of amorphous material which develop during storage of banked blood have been implicated as a cause of pulmonary micro-embolism in man following massive transfusion. Such pulmonary micro-embolism may be a causal factor in the development of post traumatic pulmonary insufficiency. At present several microaggregate filters for use in massive transfusion are commercially available, and on of these is the Fenwal filter. It is composed of a screen filter which removes microclots of 250 microns and higher, a layer of polyurethane foam and a layer of nylon wool. The resistance of this device is very acceptable and the filter may be used for several blood units, but its efficiency seems less than that of Dacron wool filter.