We have identified platelet glycoprotein (GP) Ibα as a counterreceptor for P-selectin. GP Ibα is a component of the GP Ib-IX-V complex, which mediates platelet adhesion to subendothelium at sites of injury. Cells expressing P-selectin adhered to immobilized GP Ibα, and GP Ibα–expressing cells adhered to and rolled on P-selectin and on histamine-stimulated endothelium in a P-selectin–dependent manner. In like manner, platelets rolled on activated endothelium, a phenomenon inhibited by antibodies to both P-selectin and GP Ibα. Unlike the P-selectin interaction with its leukocyte ligand, PSGL-1 (P-selectin glycoprotein ligand 1), the interaction with GP Ibα required neither calcium nor carbohydrate core-2 branching or α(1,3)-fucosylation. The interaction was inhibited by sulfated proteoglycans and by antibodies against GP Ibα, including one directed at a tyrosine-sulfated region of the polypeptide. Thus, the GP Ib-IX-V complex mediates platelet attachment to both subendothelium and activated endothelium.