The antiaggregant activity of a new benzimidazole derivative - 2,3,4,5-tetrahydro[1,3]diazepino[1,2-α]benzimidazole (DAB-15) has been investigated in vitro in comparison to reference drugs acetylsalicylic acid (ASA) and ticlid (ticlopidine) on the models of ASA, collagen, and epinephrine induced platelet aggregation in rabbits. It was established that DAB-15 exhibited dose-dependent antiaggregant activity. On the model of ASA-induced platelet aggregation, the effect of ADB-15 exceeded that of ASA and was slightly lower than that of ticlid. On the models of collagen, and epinephrine induced platelet aggregation, DAB-15 was reliably more active than the both reference drugs.