Ristocetin is an antibiotic, obtained from Nocardia lurida, previously used to treat staphylococcal infections. It is no longer used clinically because of its toxicity. It causes platelet agglutination and blood coagulation and is used to assay those functions in vitro e.g. to diagnose von Willebrand disease. Platelet agglutination caused by Ristocetin can occur only in the presence of von Willebrand factor, so if Ristocetin is added to blood lacking the factor, it will not coagulate.