Antithrombotic activity of a new benzimidazole derivative RU-891, characterized by antiaggregant activity, vs. reference drug acetylsalicylic acid was studied on the epinephrinecollagen thrombosis model in mice. Antithrombotic activity of RU-891 3-fold surpassed that of the reference drug. Histological studies showed that RU-891 significantly reduced the relative and mean areas of thrombi in sections of mouse lungs in comparison with the control.