If beta/gamma-thrombin interacted with blood plasma in vitro, shortening of recalcification period as well as partial thromboplastin time were observed in blood plasma, which were not related to low amount of alpha-thrombin in the preparation. Intensity of the procoagulant effect depended on the enzyme concentration. Presence of an excess of phospholipids did not inhibit this effect suggesting that the latter did not involve the thrombocytic factor 3 liberation. Factor V was shown to be activated by beta/gamma-thrombin in blood plasma, the rate of which depends on the enzyme concentration; the activation was not due to low content of alpha-thrombin in the preparation. Activation of factor V was only responsible for the procoagulant activity of beta/gamma-thrombin as shown by time-analysis of the reaction and by the enzyme concentrations used. Interaction of beta-gamma-thrombin with blood plasma led also to a gradual decrease in activity of factor VIII and did not affect the factor VII activity. The data obtained showed that beta/gamma-thrombin, deprived of ability to coagulate fibrinogen, maintained its activity towards the other factors of blood coagulation, suggesting the presence of independent haemostatic function in the enzyme.