SummaryVarious collagen-based materials were used to assess the structural requirements of collagen for inducing the procoagulant response of adhering platelets, as well as the collagen receptors involved. Cross-linked or monomeric collagen-related peptide (CRP), Gly-Cys-Hyp-(Gly-Pro-Hyp)10-Gly-Cys-Hyp-Gly was highly adhesive for platelets in a glycoprotein VI- (GpVI-)dependent manner. Adhesion was followed by a prolonged increase in cytosolic [Ca2+]i, formation of membrane blebs, exposure of phosphatidylserine (PS) and generation of prothrombinase-stimulating activity. Fibrils of type-I collagen were less adhesive but, once adhered, many of the platelets presented a full procoagulant response. Monomeric type-I collagen was unable to support adhesion, unless Mg2+-dependent integrin α2β1 interactions were facilitated by omission of Ca2+ ions. With all surfaces, however, post-addition of CaCl2 to adhering platelets resulted in a potent Ca2+-influx signal, followed by PS exposure and bleb formation. The procoagulant response elicited by binding to CRP was inhibited by anti-GpVI Fab fragments, but not by impeding integrin α2β1-mediated events. With fibrillar collagen, it was inhibited by blocking either the GpVI- or integrin α2β1- mediated interactions. This suggests that the triple-helical Gly-Pro-Hyp repeat in CRP and analogous sequences in fibrillar collagen stimulate the procoagulant response of adherent platelets by acting as ligands for GpVI. Influx of Ca2+ is required for this response, and adhesion via integrin α2β1 serves to potentiate the signaling effects of GpVI.