Platelets initially adhere to collagen via a divalent cation-dependent process supported by Mg2+, Mn2+, Fe2+, Cu2+, Zn2+, or Co2+ more rapidly and to a greater extent than by previously studied divalent cation-independent mechanisms. Ca2+ not only fails to support adhesion, it is inhibitory. Platelet activation and secretion are not required for adhesion by this mechanism. Monomeric and fibrillar collagens, but not denatured collagen, effectively support divalent cation-dependent adhesion. Types I, III, and IV collagen, but not type V collagen, support adhesion. A platelet surface protein of Mr 160,000, possibly identical with platelet membrane glycoprotein Ia, that binds to collagen with the appropriate divalent cation specificity has been identified and is the likely mediator of the initial divalent cation-dependent adhesion of platelets to collagen.