Conjunctive administration of the tissue-type plasminogen activator (t-PA) and the urokinase-fibrinogen covalent conjugate (UK-Fbg) was studied by the example of venous thrombosis in dogs. Comparing the effect of separate use of the two components, we observed the potentiation of thrombolytic effect induced by an iv bolus infusion administration of the tissue-type plasminogen activator (1 and 4 mg, respectively) combined with a bolus administration 15 min after the first injection of the 25,000 IU UK-Fbg. Faster-action and potentiation effects of thromboysis were observed with the same administration scheme when the t-PA was used as bolus infusion (1 and 1 mg, respectively) combined wiht a bolus of the 250,000 IU fibrinogen-modified urokinase. The findings indicate an approach to the development of efficient thrombolytic compositions.