The occurrence of spontaneous fibrinolysis has been investigated in various medical diseases, e. g. myocardial infarction, cerebral stroke, phlebothrombosis, pulmonary embolism, metastasing cancer, and others. Serum from patients has been tested for fibrinolytic break-down products by use of immunodiffusion and Immunoelectrophoresis with double-diffusion. It was anticipated that positive results might provide useful diagnostic, prognostic, and therapeutic information, particularly in patients with myocardial infarction. However, there is no apparent correlation between the occurrence of spontaneous fibrinolysis and the course of disease. In patients with haemorrhagic diathesis the tests give useful information, but in the remaining groups tested, the significance of occurring fibrinolytic break-down products cannot yet be evaluated.