The prolonged bleeding time in a 70- year-old man with a bleeding diathesis was attributed s to impaired platelet aggregation. This could be accounted for by the decrease of the platelet adhesiveness to collagen and the decreased release of adenosine diphosphate (ADP). The impaired availability of platelet factor 3 and acid phosphatase after induction by collagen was partly corrected by ADP or bovine fibrinogen. The total activity of platelet factor 3 and acid phosphatase was, however, normal in contrast to the lowered total activity of ADP and platelet factor 4, which showed even a disturbance of their release from platelets without any correction in the presence of ADP. Similarly, the total activity of β-glucuronidase was normal, though its release following collagen induction was impaired.