Blood platelets seal off leaks in the vessel wall to prevent haemorrhage by a mechanism involving their activation, adhesion to the zone of vessel injury and aggregation¹. This physiologically important haemostatic process requires Factor VIII/von Willebrand factor (vWF), a plasma glycoprotein missing in the genetic, haemorrhagic disorder called von Willebrand disease, in which platelets cannot adhere to the subendothelial layer of the vessel wall². Thus vWF is necessary for anchoring platelets to the vessel wall, and it mediates platelet–platelet interaction. This interaction requires an appropriate receptor on the platelet surface. Until recently, the antibiotic ristocetin was used to induce in vitro receptor-mediated binding of vWF to platelets and to cause their aggregation or agglutination^3–7. But although a useful laboratory tool⁸, ristocetin is clearly non-physiological, so we turned our attention to thrombin, which appears in venous blood within 45 s of venipuncture and is a very potent physiological inducer of platelet aggregation. Thrombin causes platelets to change shape from discoid to spiny spheres, and alters the platelet membrane resulting in release of ADP, serotonin and other constituents of platelet storage granules⁹. We recently showed that low concentrations (0.1–0.5 nM) of thrombin, attainable in blood, induce binding of vWF to a specific platelet receptor¹⁰. We now report that the binding of vWF to platelet receptors is also induced by ADP on its addition to platelets. Moreover, ADP released from platelet storage granules by thrombin is, at least in part, responsible for subsequent binding of ¹²⁵I-labelled vWF.