The effects of a preparation described as a human fibrinolysin were studied in vitro and in vivo to judge its possible usefulness in treating thromboembolic disease. The tests in vitro estimated the components of the plasminogen-plasmin and the coagulation system, and the results led to the expectation that the preparation would be ineffective as a thrombolytic agent in man. The tests in vivo consisted of the slow intravenous infusion of the preparation into six patients with various chronic diseases and the hourly determination thereafter of thrombolytic activity, plasma plasminogen and fibrinogen concentrations, and plasma euglobulin thrombolytic activity. The results yielded no evidence that the preparation enhanced the clot-dissolving ability of the plasma of patients.