GP Ib-IX-V–induced αIIbβ3 activation under shear flow. Whole blood from ADAP^+/+ or ADAP^−/− mice was adjusted to correct for thrombocytopenia of ADAP^−/− mouse blood and aspirated over a coverslip coated with dmA1A2 VWF, with or without fibronectin (FN), and placed in a parallel plate flow chamber. The total number of platelets tethering to the surface was measured, and the percentage of these platelets that remained stationary for 3.5 seconds or 10 seconds was determined at 20 seconds, 1 minute, or 2 minutes after the initiation of flow. Results represent stationary platelets, as a percentage of the total number of platelets contacting the surface within the sampling time, ± SEM. (A) Platelets stationary for 3.5 seconds. (B) Platelets stationary for 10 seconds. Stationary adhesion (dependent on αIIbβ3 activation) is much higher for platelets on the combined dmA1A2 VWF/FN surface than on dmA1A2 VWF alone. Results shown are the summary of at least 3 experiments. *P < .05 in a Student t test for ADAP^−/− platelets compared with ADAP^+/+ platelets.