Abstract We have developed a cell-based model of hemostasis. This model suggests that the defect in hemophilia is specifically a failure of platelet-surface factor Xa (FXa) generation, leading to a failure of platelet surface thrombin generation. Activation of FX by FVIIa/tissue factor (TF) does not compensate for a lack of FXa activation on the platelet surface by the FVIIIa/FIXa complex. This is because plasma protease inhibitors prevent FXa from moving through the fluid phase from the TF-bearing cell to the platelet surface. We have previously proposed a platelet-dependent mechanism of action for high-dose factor VIIa (FVIIa; Novoseven®, Novo Nordisk, Copenhagen, Denmark). Our data suggest that, when present at high levels, FVIIa binds to activated platelets and activates small amounts of FX independent of TF. This platelet-surface FXa can partially restore platelet-surface thrombin generation in hemophilia. Recently, van't Veer and colleagues reported results from an in vitro model in which coagulation reactions were initiated by relipidated TF. The authors concluded that high-dose FVIIa may exert a hemostatic effect in hemophilia by overcoming inhibition of FVIIa/TF activity by zymogen FVII. By contrast, we found that plasma levels of FVII did not slow thrombin generation in a model system initiated with cell-associated TF. This discrepancy highlights the potential differences between the studies of the coagulation reactions assembled on living cells compared to phospholipid vesicles. Our data suggest that in a cellular system high-dose FVIIa acts primarily by enhancing the rate of thrombin generation on platelet surfaces and not by overcoming inhibition by zymogen FVII of TF-dependent activation of FX.