Fibrinogen and fibrin sedimentation by different protamine sulphate preparations have been studied. Ionic strength and protamine sulphate concentration are found to influence the sedimentation reaction (paracoagulation). High sedimentation activity is inherent in protamine sulphate preparations with the lower electrophoretic mobility, that is with the higher molecular weight. The protamine sulphate reaction with fibrinogen and fibrin is of electrostatic character as the long polycationic chain of protamine is coupled with the negatively charged loci either of fibrin or of fibrinogen molecules, thus evoking aggregation. In this case the fibrin molecules being brought together favour the specific mutual binding due to the active sites of polymerization, specific fibres or gel being formed.