The following conclusion were obtained from the studies on the mechanism of the development of hemorrhagic tendency which occurred in the case of compatible preserved blood transfusions. Following the repeated blood transfusions of small quantity, fibrinolysis, capillary permeability promoting action and increased heparinlike substance in blood were observed. These findings are manifestation of alarm reaction, and, in this way, potencial hemorrllagic tendency is considered to occur. Influence of citric acid, an anticoagulant of preserved blood on hemorrhagic tendency were studied. Elevated citrate levels were found following massive transfnsions of preserved citrated blood. However, it seems to be no correlation between the occurrence of hemorrhagic tendency and elevated citrate levels. Elevated citrate levels cause circulatory derangement and tetany rather than hemorrhagic tendency.