Commercial aurin tricarboxylic acid (ATA) has been reported to interfere specifically with von Willebrand factor-glycoprotein Ib (vWF-GPIb) axis. This study was designed to explore the antithrombotic effects of ATA by examining its effects on guinea pig platelet function in vitro, in vivo and ex vivo. In vitro, addition of various concentrations of ATA to platelet-rich guinea pig plasma totally inhibited ristocetin-induced platelet aggregation, as expected. Unexpectedly, however, ATA similarly inhibited the aggregation induced by ADP, PAF, collagen, I-BOP (a thromboxane A2/prostaglandin H2 analogue) and arachidonic acid. In vivo, the antithrombotic action of ATA was assessed in a model of acute platelet-dependent guinea pig mesenteric artery thrombosis triggered by laser-induced intimal injury. As the thrombotic response of arteries to such injury is a spontaneous cyclic recurrent process, 5 arteries in each animal were consecutively studied for 15 min each after i.v. bolus injection of 5, 7.5 or 10 mg/kg of ATA, which reduced the number of recurrent thrombi per artery in a dose-dependent manner. The highest dose of 10 mg/kg induced maximal inhibition of thrombus formation (72%, p < 0.001) 5 min after injection. Ex vivo, platelet aggregation was assessed in blood samples taken before and after i.v. bolus injection of 10 or 15 mg/kg ATA. Ten mg/kg significantly inhibited collagen-induced aggregation, and 15 mg/kg, the aggregation induced by ristocetin, ADP, PAF, collagen, I-BOP and arachidonic acid. The results of the in vivo studies confirmed that ATA is an effective antithrombotic agent. In the in vitro and ex vivo studies, ristocetin-induced platelet aggregation confirmed that ATA interacts with the vWF-GPIb axis, and suggests that the final common pathway of the aggregation induced by other agents tested consists of fibrinogen binding to the platelet GPIIb/IIIa receptor. We conclude that ATA interferes with vWF binding to GPIb, that it may interact with fibrinogen binding to GPIIb/IIIa, and that it might possess potent antithrombotic properties in platelet-mediated thrombosis.