At present, thromboembolic diseases represent a major cause of morbidity and mortality. An important therapeutic advance in the management of these problems was achieved by the introduction of anticoagulants. However, their use is designed to restrict extension of an already existing thrombus or to diminish the incidence of new thrombi or emboli; they have little, if any, effect on the immediate consequences of an acute thromboembolic episode. For this reason, much attention is being given to the development of thrombolytic (clot-dissolving) agents which could effect the rapid resolution of thrombi or emboli acutely formed within the vascular tree. Restoration of the circulation through the affected vessel might then limit the damage caused by vascular obstruction, and dissolution of the clot would remove a source of further embolization. This new approach seems entirely feasible since fibrin, the major insoluble constituent of the clot, is susceptible to enzymatic lysis, and agents capable