Abstract: The existence of high fibrin-affinity urokinase is discovered by an isolation procedure using fibrin precipitated on an adsorptive-solid matrix. By the method described, the high affinity form of plasminogen activator can be isolated directly from urine or from kidney tissue culture medium. The method is economical and provides a relatively high yield of the activator. The high affinity that this plasminogen activator has for fibrin is a property that makes it an improved thrombolytic agent and when radiolabelled provides a new diagnostic agent for the specific detection of fibrin thrombi through nuclear scanning. The newly-isolated plasminogen activator has the following characteristics: a molecular weight of about 56,000 Daltons, a specific activity of about 40,000-50,000 CTA units/mg, the appearance of a single chain structure and a high affinity for fibrin.