The influence of increased fibrinolysis on bleeding tendency was studied. In acute leukemia and aplastic anemia, the co-existence of high fibrinolytic activity and thrombocytopenia was frequently observed in hemorrhagic cases. In fibrinolytic state, the bleeding tendency was more prominent in thrombocytopenic animals. In microscopic studies of mesenteric vessels of the rabbits, no bleeding was found even after activation of fibrinolysis or thrombocytopenia was induced. When the small vessels were previously injured with mechanical or chemical stimuli, a massive hemorrhage ensued. It is concluded therefore that vascular injury is an important factor in the mechanisms of hemorrhage even in elevated fibrinolytic state and that thrombocytopenia enhances the fibrinolytic bleeding.