Experimental studies on the qualitative changes in platelets following massive transfusions of preserved blood were performed. It was observed that the platelet adhesiveness was decreased and the platelet agglutination was tended to increase, surmising a kind of relation between those phenomena and the distribution change of platelets. Increase in resistance of platelets, decrease in thromboplastinogenase and remarkable suppression in clot retraction were observed only after massive transfusions of preserved blood.