An elevated plasma fibrinogen level is a risk factor for thrombotic cardiovascular disease, but which of fibrinogen's functions is responsible for the increased risk is unknown. To define better the contribution of fibrinogen to large vessel thrombus formation, we studied carotid artery thrombosis in wild-type mice, mice lacking fibrinogen (fbg^–/–), mice treated with 7E9 (a blocking antibody to the fibrinogen γ-chain C-terminus), and mice expressing a mutant fibrinogen (γΔ5) that lacks the γ-chain platelet-binding motif QADGV. In control mice, thrombus formation resulted in occlusion in 8 ± 2 minutes (mean ± SD). In fbg^–/– mice, thrombi grew to large sizes, but then they abruptly embolized, confirming previous observations by others in an arteriolar thrombus model. In contrast, mice treated with 7E9 and γΔ5 mice developed only small, nonoclusive mural thrombi and embolization was limited. These findings reveal that a fibrinogen antibody, 7E9, or a fibrinogen mutant retaining clotting function, can limit thrombus formation more effectively than the complete absence of fibrinogen. We hypothesize that the smaller thrombi in these animals result from the ability of fibrin to bind and sequester thrombin and/or the ability of the altered fibrinogen molecules, which cannot recruit platelets, to bind to and passivate the surface.