Summary. To explore the possible role of a residual or variant αIIbβ3 integrin (αIIbβ3) in thrombogenesis, we used a new ex vivo perfusion chamber model to examine blood from patients with different subtypes of Glanzmann's thrombasthenia (GT). Non-anticoagulated blood was perfused through capillaries coated with type III collagen for 4·5 min (shear rate: 1600/s). Platelet deposition was quantified as platelet adhesion and mean thrombus size volume; fibrin and von Willebrand Factor (VWF) were specifically revealed by immunohistochemistry. In two patients with variant and in one patient with type II GT, platelet adhesion was maximal and we observed an unexpected formation of thrombi that were smaller than normal in size. These thrombi were surrounded by a thick meshwork that displayed a strong staining for fibrin and VWF. In two patients with heterozygous GT, platelet adhesion and thrombogenesis were normal. In two patients with type I GT, there was no thrombus formation, although platelet adhesion was also maximal. These data suggest the existence of a substitute pathway for thrombogenesis mediated by fibrin and possibly αIIbβ3 (αIIbβ3 at a reduced level, as in type II, and/or abnormal) as this fibrin network was not observed in type I GT with no αIIbβ3. These interactions might facilitate haemostasis and even lead to thrombosis under certain favourable conditions. Furthermore, these data might have pharmacological relevance to the development of anti-αIIbβ3 antithrombotic agents.