We have purified the platelet membrane glycoprotein Ia-IIa complex by detergent solubilization and sequential affinity chromatography on Concanavalin A-Sepharose and collagen-Sepharose. The complex, which is identical to the VLA-2 complex of lymphocytes and other cells and contains subunits of 160 and 130 kD on SDS-PAGE, was labeled with 125I and incorporated into phosphatidyl choline liposomes. The liposomes, like intact platelets, adhered to collagenous substrates in an Mg++-dependent manner with a K'a(Mg++) of 3.5 mM. Little adhesion of the liposomes to collagen occurred when Mg++ was replaced by Ca++ or EDTA. Calcium ions inhibited the Mg++-dependent adhesion with a K'i(Ca++) of 5.5 mM. Liposomes containing the Ia-IIa complex adhered to substrates composed of types I, II, III, and IV collagen, but did not effectively adhere to substrates composed of type V collagen or gelatin. Adhesion to collagen was specific. The liposomes did not adhere to fibronectin, vitronectin, laminin, thrombospondin, fibrinogen, or von Willebrand factor substrates. The monoclonal antibody P1H5, which specifically immunoprecipitated the Ia-IIa complex, also specifically inhibited the Mg++-dependent adhesion of both platelets and Ia-IIa-containing liposomes to collagen substrates. These findings provide additional evidence that the platelet membrane Ia-IIa complex is the mediator of Mg++-dependent platelet adhesion to collagen and suggest that the VLA-2 complex may also function as an Mg++-dependent collagen receptor in other cells.