Previous studies have demonstrated that fibrin clot quality is dictated by the concentration of thrombin at the time of gelation. These studies were conducted by adding a single concentration of thrombin to a solution of fibrinogen; however, in vivo, thrombin generation proceeds on a cell surface, where the concentration of thrombin changes during the reaction. Surprisingly, few studies have examined how thrombin generation on a cell surface influences clot formation, structure, and stability. The present work is a study of the cellular properties that influence thrombin generation and the ways in which these properties contribute to clot formation and stability. Patients with hemophilia form clots that have increased fibrinolytic susceptibility due to reduced rates of thrombin generation. With the goal of improving the rate of thrombin generation in these patients, we explored whether the combination of rehydrated, lyophilized (RL) platelets (a cellular component) and recombinant factor VIIa (rFVIIa, a drug used to treat hemophilia) improved thrombin generation and the quality of the fibrin network over rFVIIa alone. Using an in vitro model of coagulation, we determined that RL platelets combined with 10 nM rFVIIa improved thrombin generation and clot stability to levels seen with 50 nM of rFVIIa alone. We further determined that the mechanism responsible for this improvement involved the procoagulant nature of RL platelets, suggesting cellular procoagulant activity can play a significant role in thrombin generation and the quality of the fibrin network. We next focused on the role of vascular cells in thrombin generation and demonstrated that intravascular endothelial cells support low levels of thrombin generation, which results in the formation of a loose network of fibrin. However, extravascular cells (fibroblasts and smooth muscle cells), support significantly higher rates of thrombin generation, which produce tighter fibrin networks. Modulation of the endothelial cell procoagulant phenotype through cytokine stimulation caused thrombin generation and clot quality to shift to that observed for extravascular cells, suggesting a novel link between inflammation and thrombosis. These studies provide a better understanding of the mechanisms through which thrombin generation on a cell surface ultimately affects the quality of the fibrin clot. Our findings will likely enhance our ability to design therapeutics for bleeding and thrombotic disorders. Advisors/Committee Members: Campbell, Robert A., Wolberg, Alisa, University of North Carolina at Chapel Hill.