The purpose of the present study was to investigate and to compare the interactions of platelets and proteins in flowing non-anticoagulated human blood with the biomaterials polyethylene-terephthalate (Dacron) and polytetrafluoroethylene (PTFE, Teflon). The respective biomaterials were positioned in a parallel-plate perfusion chamber, and exposed to flowing blood for 5 min at wall shear rates characteristic for veins (100/s), medium sized (650/s) and moderately stenosed arteries (2,600/s). Blood-material interactions were morphologically quantified as platelet-surface adhesion, thrombus volume and fibrin deposition. Platelet adhesion to Dacron was highest at the lowest shear rate (13%) and decreased with increasing shear (4% at 2600/s). In contrast, platelet adhesion to PTFE was shear rate independent (17-19%), and significantly higher than the adhesion to Dacron at 2600/s (P < 0.05). A hallmark of the platelets adherent to PTFE and Dacron was the large percentage of platelets not spread out on the surface. This indicates that both materials were poor platelet activators, even though immunostaining demonstrated the adsorption of the platelet adhesive proteins von Willebrand factor and fibronectin. Adsorption of fibrinogen was also prevailing on both materials. Virtually no thrombi formed on Dacron, while a few small platelet thrombi were observed on PTFE. Less than 1% of the Dacron and PTFE surfaces were covered by fibrin, irrespective of the shear rate. Thus, Dacron and PTFE interact differently with flowing non-anticoagulated human blood, and Dacron is apparently the least thrombogenic material.