CD42d is an 83 kD surface glycoprotein that non-covalently associates with GPIb and GPIX to form a receptor complex for von Willebrand factor on megakaryocytes and resting platelets. Binding sites for von Willebrand factor and thrombin have been localized to the GPIba chain of the GPI-b-V-IX complex. Platelet activation with thrombin cleaves the GPI-b-V-IX complex to release a 69 kD soluble fragment. Presence of the GPI-b-V-IX complex is important in Bernard-Soulier syndrome, a rare bleeding disorder.