Microaggregates are formed during the storage of human blood and are composed largely of platelets and leukocytes. These microparticles reside in the buffy coat fraction of blood. The formation of microaggregates can be successfully prevented by removal of the buffy coat or by treatment of blood with drugs which inhibit platelet function prior to storage. Once formed, the volumes of microaggregates in aged blood can be significantly reduced by washing, centrifugation or treatment with urokinase or streptokinase. Glycerol frozen red blood cells and blood components--packed red blood cells and plasma--are free of microaggregates, and they can be infused without fear of embolic consequence.