1) One tenth volume of 2000U/ml UK was added to the normal human plasma kept at 37°C. The decrease of fibrinogen in this plasma was approximately 50mg/ml even after 60min. incubation, but the fibrin clot lysis time was completed within 15min., when aliquots were taken from the mixture successively for the assay of plasma fibrin clot lysis. On the contrary, the grade of digestion of fibrinogen and fibrin by plasmin proceeded at the same rate when purified fibrinogen and plasminogen were used.It was suggested that the digestion between fibrinogen and fibrin in norml human plasma was differentiated by the presence of plasmin inhibitors. The same differentiating digestion was observed with the use of purified fibrinogen, plasminogen and α2-macroglobulin or Trasylol as the plasmin-inhibitor, whereas it was not with t-AMCHA as the activator-inhibitor.2) Immediate as well as progressive inhibitor activity in normal human plasma was assayed by fibrin clot lysis time method and fibrinogenolysis method. It was found that the immediate activity was 15CTAU/ml and the progressve one was 36CTAU/ml by fibrinogenolysis method, while the former was 8CTAU/ml and the latter one was only 5CTAU/ml by fibrin clot lysis time mothod. The result indicated that the plamin inhibitor, immediate as well as progressive, acts more intensively on the process of fibrinogenolysis than fibrinolysis.3) Normal human plasma was chromatographed on Sephadex G200 column and the inhibitor activities in the eluted fractions were assayed by both fibrin clot lysis and fibrinogenolysis methods. It was revealed that the immediate inhibitor activity on fibrinolysis was eluted in three-peaked profile and the most of the activity was localized in the eluttion region of α2-M, while the activity of progressive inhibitors formed three-peaked profile with the same activity between them. With the use of fibrinogenolytic method, it was revealed that the immediate form were eluted in two-peaked profile, while the progressive form was eluted in probably in four-pearked profile in which the highest activity was localized in the region of α1-antitrypsin.It was concluded that the differentiating digestion between fibrinogen and fibrin in plasma was performed at the presence of plasmin inhibitors. The main role of fibrinolysis inhibition was played by α2-M, while that of fibrinogenolysis by α1-antitrypsin.