Effect of RO318220 on platelet-supported thrombin generation in the absence of added factor Va. Thrombin generation assays were performed with aspirin-treated, washed, and gel-filtered platelets incubated with vehicle or inhibitor and activated with (A) α thrombin (0.2 U/mL; closed symbols) or (B) SFLLRN (50 μM; open symbols). Thrombin generated was assayed by S-2238 hydrolysis as described in “Materials and methods.” Squares indicate control DMSO-treated platelets; circles, RO318220-treated (20 μM) platelets (3 experiments); and triangles, RO318220-treated (50 μM) platelets (6 experiments). Error bars represent SEM.