To evaluate the involvement of the glycoprotein (GP) IIb/IIIa–dependent process in platelet deposition and thrombus growth on capillaries coated with human type III collagen, the effects of incremental doses of Lamifiban, a potent specific synthetic GPIIb/IIIa antagonist, were studied in ex vivo capillary perfusion chambers using guinea pig blood. In this model, nonanticoagulated blood was perfused for 4.5 minutes at three shear rates: 100, 650, and 1600 s⁻¹. Platelet deposition was quantified by computer-assisted morphometry and expressed as platelet adhesion (percentage of capillary surface covered with spread and contact platelets and platelets implicated in thrombus), mean thrombus height, and total thrombus cross-sectional area. In control untreated guinea pigs, platelet adhesion and thrombus height were 63% and 2.5 μm at 100 s⁻¹, 60.5% and 13.8 μm at 650 s⁻¹, and 45% and 28.1 μm at 1600 s⁻¹, respectively. At 100 s⁻¹, Lamifiban had no effect on platelet deposition at any of the three doses administered to the guinea pigs (0.3, 1, and 3 mg/kg). At 0.3 mg/kg and shear rates of 650 and 1600 s⁻¹, Lamifiban had no effect on platelet adhesion or thrombus size, but at 1 and 3 mg/kg and shear rates of 650 and 1600 s⁻¹, it significantly reduced thrombus size. At 1600 s⁻¹, 1 mg/kg Lamifiban significantly increased platelet adhesion from 45% to 62.5%, whereas at 3 mg/kg it induced a significant overall decrease from 45% to 25% and qualitatively increased the ratio of contact to spread platelets. These data suggest that at high shear rates, GPIIb/IIIa participates in platelet spreading and that there is a balance between platelet involvement in adhesion to the thrombogenic surface and the growth of the already formed thrombus. This indicates that important clinical implications of an optimal therapeutic degree of GPIIb/IIIa antagonism could be expected.