Summary Studies of platelet function in four kindreds affected with the Bernard‐Soulier Syndrome are summarized. Platelet aggregation induced by ADP and by dilute collagen suspensions was normal, but the increase in optical density and platelet “volume” that normally proceeds aggregation was lacking. Platelet aggregation by bovine fibrinogen was totally deficient, and the rate of aggregation by both ADP and collagen was abnormally rapid. The serum prothrombin time was abnormal, but other tests of platelet factor 3 activity were normal, as were estimates of platelet adhesiveness. The lack of initial platelet “swelling” and deficient bovine fibrinogen‐induced platelet aggregation are heretofore unrecognized features of this disorder that may provide information as to the nature of the underlying abnormality.