The role of the tendon of the long head of the biceps in the stabilization of the head of the humerus was studied in 15 freely hanging anatomic specimen shoulders. Upward migration of the humeral head was measured by noting any reduction in the acromiohumeral distance in roentgenograms of the shoulder. There was a statistically significant decrease in the acromiohumeral interval on tensing the short head of biceps, but there was no significant change in the interval on tensing either the long head or both heads of the biceps brachii. Severing the tendon of the long head while both the heads were tensed caused a significant upward migration of the head of the humerus. One of the important functions of the long head of the biceps is to stabilize the humeral head in the glenoid during powerful elbow flexion and forearm supination by the main muscle. Sacrifice of the intraarticular segment of this tendon in surgical procedures of the shoulder may produce instability and dysfunction.