The platelet response to collagen is a primary event in hemostasis and thrombosis, but the precise roles of the numerous identified platelet collagen receptors remain incompletely defined. Attention has recently focused on glycoprotein VI (GPVI), a receptor that is expressed on platelets in association with a signaling adapter, the Fc receptor gamma chain (Fc R␥). Genetic and pharmacologic loss of GPVI function results in loss of collagen signaling in platelets, but studies to date have failed to demonstrate that GPVI-Fc R␥ expression is sufficient to confer collagen signaling responses. These results have led to the hypothesis that collagen responses mediated by GPVI-Fc R␥ may require the collagen-binding integrin ␣21 as a co-receptor, but this model has not been supported by a recent study of mouse platelets lacking ␣21. In the present study we have used a novel anti-GPVI monoclonal antibody to measure the level of GPVI on human platelets and to guide the development of GPVI-expressing cell lines to assess the role of GPVI in mediating platelet collagen responses. GPVI receptor density on human platelets appears tightly regulated, is independent from the level of ␣21 expression, and significantly exceeds that on previously characterized GPVI-expressing RBL-2H3 cells. Using newly generated GPVI-expressing RBL-2H3 cells with receptor densities equivalent to that on human platelets, we demonstrate that GPVI expression confers both adhesive and signaling responses to collagen in a graded fashion that is proportional to the GPVI receptor density. These results resolve some of the conflicting data regarding GPVIcollagen interactions and demonstrate that 1) GPVI-Fc R␥ expression is sufficient to confer both adhesion and signaling responses to collagen, and 2) GPVI-mediated collagen responses are receptor density-dependent at the receptor levels expressed on human platelets.The response of platelets to vessel wall injury is a primary event in arterial thrombosis (1). Platelets respond to vessel wall injury via surface receptors that recognize exposed subendothelial matrix proteins, the most abundant of which is collagen. Collagen is unusual among platelet ligands because it mediates both platelet adhesion and platelet activation (2). Platelet adhesion to collagen initiates the thrombotic response, and platelet activation in response to collagen accelerates the thrombotic response through release of granule contents and activation of platelet integrins. The ability of collagen to mediate both of these steps suggests that the platelet response to collagen may be a key step in the regulation of arterial thrombosis. A molecular understanding of the platelet receptors that mediate collagen responses is therefore likely to shed light on the pathogenesis of common vascular diseases such as myocardial infarction and stroke.The receptors for collagen on the surface of platelets may be divided into those which interact indirectly through collagenbound von Willebrand factor (vWF), 1 including GPIb␣ and the integrin ␣ IIb  3 ...