The ability of factor VIIa to initiate thrombin generation and clot formation in blood from healthy donors, blood from patients with hemophilia A, and in anti-factor IX antibody-induced ("acquired") hemophilia B blood was investigated. In normal blood, both factor VIIa-tissue factor (TF) complex and factor VIIa alone initiated thrombin generation. The efficiency of factor VIIa was about 0.0001 that of the factor VIIa-TF complex. In congenital hemophilia A blood and "acquired" hemophilia B blood in vitro, addition of 10 to 50 nM factor VIIa (pharmacologic concentrations) corrected the clotting time at all TF concentrations tested (0-100 pM) but had little effect on thrombin generation. Fibrinopeptide release and insoluble clot formation were only marginally influenced by addition of factor VIIa. TF alone had a more pronounced effect on thrombin generation; an increase in TF from 0 to 100 pM increased the maximum thrombin level in "acquired" hemophilia B blood from 120 to 480 nM. Platelet activation was considerably enhanced by addition of factor VIIa to both hemophilia A blood and "acquired" hemophilia B blood. Thus, pharmacologic concentrations of factor VIIa cannot restore normal thrombin generation in hemophilia A and hemophilia B blood in vitro. The efficacy of factor VIIa (10-50 nM) in hemophilia blood is dependent on TF.