Role of platelet GpIb-V-IX in fibrin formation under flow. Platelets were superfused with plasma at 250 s⁻¹ in the presence (10 μg/mL) of isotype control mAb 2D4 or one of the following anti-GpIbα mAbs: 2D2 (block of thrombin binding site), 12G1 (block of shear-induced VWF binding) or 6B4 (block of shear- and ristocetin-induced VWF binding). (A) Representative microscopic images (Nikon Diaphot 200 microscope, see “Methods”) were taken at 0-60 seconds after start of fibrin formation: phase contrast and OG488-fibrin(ogen) (bars, 10 micron). (B) Lag time to start of fibrin formation. (C) Quantification of fibrin formation from subtracted images (60 seconds). (D) Staining with OG488-fibrin(ogen). (E) Fractions of platelets positive for OG488–annexin A5. Means ± SE; n = 3-4; ***P < .001 vs control mAb.