Mechanisms of the anticoagulant activity of p-aminobenzoic acid (PABA) were studied. The specific antithrombin activity of PABA is aIIa = 7.00 +/- 0.32 IU/mg and the specific antiactivated factor Xa activity is aXa = 6.70 +/- 0.12 IU/mg. Study of the antithrombotic activity of PABA in rats with a model of venous stasis showed that intravenous injection of PABA at a dose of 1.5 mg/kg prevented the experimental thrombosis development 1.5 h after injection. The activity exhibited a peak 3 h after drug injection and ceased by the 5th hour. Equal antithrombotic activity in the rat blood plasma was observed for fraxiparin at a dose of 40 aXa IU/kg and PABA at 25 aXa IE/kg (1.5 mg/kg). At the same time, PABA affected neither the number of thrombocytes nor their response to the thrombocyte aggregation factors (ADP or adrenaline).