Effect of RO318220 on microvesicle-supported thrombin generation. Thrombin generation was assayed in the presence of 1 nM factor Xa, 20 nM factor Va, and 1 μM prothrombin using the complete activation mixture (▦) or the 800g supernatant (▪) of aspirin-treated, washed, and gel-filtered platelets (half of which were preincubated with 100 μM calpeptin) before being incubated with vehicle, 20, or 100 μM RO318220 and activated with SFLLRN (30 μM). Thrombin generation was assayed by S-2238 hydrolysis as described in “Materials and methods.” Activities associated with the platelet surfaces (□) were calculated by subtraction of supernatant activities from those of the complete mixtures. Means and standard errors were derived from 8 experiments.