Blood stored at 4 C in ACD or CPD solution develops microaggregates composed primarily of fibrin and platelets. This debris has been implicated in the pathogenesis of posttraumatic pulmonary insufficiency in man. Recent work indicates that gentle agitation of the blood during storage appears to decrease debris formation. These studies were undertaken to establish more clearly the effect of agitation on debris formation. Blood was drawn in CPD from healthy young males, non-aspirin ingesting donors and stored at 4 C. One-half of the bags were gently and continuously agitated for 21 days and the other half remained stationary. At the end of the storage period, platelet counts and screen filtration pressures were measured. Agitated blood showed significantly less debris formation and significantly higher platelet counts. Gentle agitation was shown to be an effective method for preventing debris formation in banked blood.