The platelets of three patients with the hereditary giant platelet syndrome of Bernard and Soulier failed to aggregate in response to either ristocetin or bovine fibrinogen. The results of aggregation experiments using mixtures of platelets and plasma suggest that a reaction between a plasma factor deficient in von Willebrand's disease and a platelet component lacking in our patients, and leading to platelet aggregation independently of adenosine diphosphate (ADP), is essential for normal haemostasis.