A high-sensitive method is developed for determining the degree of plasmin-catalyzed fibrinogen hydrolysis by the released amino groups stained with trinitrobenzene sulphoacid. The method permits determining 0.02-0.08 casein units of plasmin. The method made it possible to establish that after streptokinase activation plasmin hydrolyzes equally fibrinogen and fibrin in solution and as gel. When a tissue activator is used, fibrin intensifies significantly the plasminogen activation. Inhibition of plasmin by an inhibitor produced from soya is considerably slowed down in fibrin gel.