Earlier studies have shown that patients with von Willebrand’s disease are considered to have normal platelets, but they lack at least one plasma protein. Results are presented indicating that ristocetin, known to aggregate normal platelets, fails to cause platelet aggregation in the group of von Willebrand’s disease patients exhibiting no platelet adhesiveness. It is postulated that there may be two groups of patients within von Willebrand’s disease and, further, that ristocetin will provide a useful approach to study the plasma deficiency or abnormality in von Willebrand’s disease.

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