The course of defibrination after administration of a thrombin-like snake venom protease, Defibrase®, has been studied in two patients with arterial occlusive disease. Agarose gel filtration of fresh plasma makes it possible to differentiate between high (HMWD) and low molecular weight fibrinogen and fibrin derivatives (LMWD) which arise during therapy. HMWD can be quickly and dependably detected in the column eluate by the staphylococcal clumping test. They probably consist mainly of fibrin monomer complexes during the coagulation phase, but during the fibrinolytic phase mainly of high molecular weight fibrinolytic degradation products. LMWD in the column eluates have been identified by radial immunodiffusion. By combining several different methods for the determination of fibrinogen-fibrin-related material the process of defibrination and reactive fibrinolysis may be studied in detail. The protracted activity of one injection of Defibrase can be clearly documented