Ristocetin A is used clinically as a tool for the measurement of von Willebrand protein due to its ability to induce the aggregation of platelets in plasma being proportional to the concentration of von Willebrand protein. The enzymatically induced cleavage ofα- l-rhamnose from the tetrasaccharide of ristocetin A removes this ability to induce platelet aggregation in plasma, providing a possible insight into the structural basis for this effect.