GPVI supports platelet tethering and stable adhesion to fibrin in a flow system. (A-D) Hirudinated mouse or human whole blood was perfused through fibrin-coated microcapillaries at 300 s⁻¹, monitored for 3 minutes, and platelet adhesion observed by real-time video microscopy. The bar graph represents the quantification of (A-B) platelet tethering to fibrin and (C-D) the adhesive behavior of platelets adherent to fibrin (D, detachment; R, rolling; S, stationary adhesion) expressed as the percentage of the total adherent platelets. (A,C) Blood from WT (n = 5) or GPVI^−/− (n = 5) mice; (B,D) human blood treated with a control Fab (n = 4) or the anti-GPVI Fab 9O12 (n = 4). (E) Hirudinated human whole blood treated with a control Fab or the Fab 9O12 was perfused through fibrin-coated microcapillaries at various wall shear rates: 150, 500, and 1500 s⁻¹. Platelet adhesion was monitored by DIC microscopy and adherent platelets were counted after 3 minutes. Mean ± SEM, *P < .05, **P < .01, Mann-Whitney U test.