Ristocetin is a glycopeptide antibiotic, obtained from Amycolatopsis lurida, previously used to treat staphylococcal infections.

It is now used solely to assay those functions in vitro in the diagnosis of conditions such as von Willebrand disease (vWD) and Bernard–Soulier syndrome.

Through an unknown mechanism, the antibiotic ristocetin causes von Willebrand factor to bind the platelet receptor glycoprotein Ib (GpIb), so when ristocetin is added to normal blood, it causes agglutination.

This increased binding causes vWD because the high-molecular weight multimers are removed from circulation in plasma since they remain attached to the patient's platelets.

Thus, the ristocetin cofactor activity depends only upon high-molecular multimers of the factor present in circulating plasma.