This review deals with aspects of fibrinolysis in which significant developments have taken place in the last few years. The structural changes of plasminogen during its activation are now identified precisely; the recent description of a thrombotic tendency in a kindred characterized by a defect of this protein emphasizes its important role in the homeostatic balance. Several activators of plasminogen are now identified; some of them, such as tissue and vascular activators, appear to have an important role in physiology and pathology. The recent characterizations of the alpha 2-antiplasmin and of antiactivators have widened our understanding of the inhibitors of fibrinolysis: a defect of the plasmin inhibitor seems to be associated with an haemorrhagic tendency, whereas high antiactivator levels were encountered in thrombotic conditions. The clinical use of fibrinolytic agents appears to be promising in conditions such as recurrent deep vein thrombosis and in the post-phlebitic syndrome. Thrombolytic therapy with urokinase or streptokinase appears to have elective indications in patients with acute deep vein thrombosis and massive life-threatening pulmonary embolism.