Cardiovascular disease is the leading cause of death and disability worldwide. This dissertation explores the role of the clotting factor fibrinogen and red blood cells (RBCs) to arterial thrombosis. Elevated plasma fibrinogen is associated with arterial thrombosis in humans and directly promotes thrombosis in mice, but the contribution of the γA/γ′ fibrinogen isoform to thrombosis is controversial. To determine if γA/γ′ is prothrombotic, we separated γA/γA and γA/γ′ from human plasma and determined the effects on in vitro clot formation and on in vivo thrombus formation. Both γA/γA and γA/γ′ were cleaved by murine and human thrombin and were incorporated into murine and human clots. When γA/γA or γA/γ′ was spiked into plasma, γA/γA increased the fibrin formation rate to a greater extent than γA/γ′. In mice, compared to controls, γA/γA infusion shortened the time to carotid artery occlusion, whereas γA/γ′ infusion did not. Additionally, γA/γ′ infusion led to lower levels of plasma thrombin-antithrombin complexes following arterial injury, whereas γA/γA infusion did not. These data suggest that γA/γ′ binds thrombin in vivo and decreases prothrombotic activity. Together, these findings indicate that elevated levels of γA/γA promote arterial thrombosis in vivo, whereas γA/γ′ does not. RBCs are the most abundant cell type in blood and increased hematocrit is associated with thrombosis. While it is known that RBCs support thrombin generation and increase platelet activation and aggregation, the specific mechanism by which RBCs influence clotting is unclear. In reconstituted human blood ex vivo, RBCs dose-dependently increased thrombin generation in the absence of platelets, although effects were blunted or absent in the presence of platelets. Compared to controls, mice infused with RBCs formed thrombi at a faster rate and had a shortened time to vessel occlusion in a carotid artery injury model. Interestingly, there was no difference in circulating thrombin-antithrombin complexes between RBCHIGH and control mice, and thrombi did not differ in size or fibrin content, suggesting elevated hematocrit promotes arterial thrombosis by a thrombin-independent mechanism. Our data suggest that reducing hematocrit may reduce arterial thrombosis in humans. Future experiments will investigate the RBC effect on platelets to thrombosis. Advisors/Committee Members: Walton, Bethany, Wolberg, Alisa, Coleman, William, Cooley, Brian, Key, Nigel, Parise, Leslie, Wolberg, Alisa.