Platelets were found to provide a surface for activation of plasminogen by the tissue-type plasminogen activator (t-PA) at an optimum concentration and to potentiate the generation of plasmin by the amidolytic method, fibrin lysis time and fibrin plate method. The effect of platelets on amidolytic activity on S-2251 was due to the potentiating effect of plasminogen activation by t-PA, because it was observed only in the presence of both plasminogen and t-PA. Plasmin generation was also evidenced in the SDS-PAGE profile of the supernatant from a mixture containing t-PA and plasminogen with platelets. These findings suggests that the potentiating activity of platelets on plasminogen activation by t-PA in circulation is one of the causes of fibrinogenolysis during fibrinolytic therapy with a high dose of t-PA. Platelets from patients with various diseases showed different potentiating activity on plasminogen activation by t-PA. The assay of this ability of platelets may be a new tool for evaluating their role in the blood fibrinolytic process.