P latelet aggregation contributes to arresting bleeding at wound sites, but may cause occlusion of atherosclerotic vessels, thus curtailing blood flow to vital organs. According to current dogma, the integrin alphaIIbbeta3 plays an exclusive role in linking platelets to one another through interactions with fibrinogen or vWf. We demonstrate here that, depending on shearing flow conditions, this process may require vWf binding to glycoprotein Ibalpha, even when alphaIIbbeta3 is competent to bind adhesive ligands. Platelet activation induced solely by high shear stress is initiated by glycoprotein Ibalpha interaction with vWf, but results in aggregation only if the latter can bind concurrently to alphaIIbbeta3. In contrast, platelets exposed to high shear rate after activation by exogenous agonists such as ADP and epinephrine can aggregate when fibrinogen is the alphaIIbbeta3 adhesive ligand, yet only if vWf binding to glycoprotein Ibalpha can also occur. Thus, the latter interaction appears to provide a bond with biomechanical properties necessary to overcome the effects of high shear rate and initiate interplatelet cohesion. These findings highlight the distinct function of two adhesive receptors mediating platelet aggregation under varying fluid dynamic conditions, and modify the current interpretation of a crucial event in hemostasis and thrombosis.