Each of three distinct, concentric layers of human arterial thrombi, was analysed immunochemically for the plasminogen and fibrin content, and for the ex vivo susceptibility to thrombolysis by various thrombolytic agents in a saline or plasma milieu. The age of the thrombus layer determined: (a) the plasminogen content; (b) the fibrin content, inferred from the recovery of fibrin degradation products after complete lysis and from the binding of a monoclonal anti-fibrin antibody in a perfusion system, and (c) the lysibility of the thrombus. Plotting concentration of the various thrombolytic agents against percentage of lysis at several time points allows for reading of equivalent potencies of the respective units. Undiluted solutions of APSAC and rt-PA, prepared according to the manufacturer's directions, were less effective than diluted solutions, which has consequences for local therapy. All agents were at least as effective in saline as in a plasma milieu. We conclude that the plasminogen content of aged arterial thrombi is sufficient for complete and rapid thrombolysis. Only after several months do fibrin and plasminogen become so far degraded or replaced that the thrombi become resistant to thrombolysis. Chemicals/CAS: alteplase, 105857-23-6; anistreplase, 81669-57-0; plasminogen, 9001-91-6; saruplase, 99149-95-8; streptokinase, 9002-01-1; tissue plasminogen activator, 105913-11-9; urokinase, 139639-24-0; Anistreplase, 81669-57-0; Fibrin Fibrinogen Degradation Products; Fibrin, 9001-31-4; Fibrinolytic Agents; Plasminogen, 9001-91-6; Recombinant Proteins; Sodium Chloride, 7647-14-5; Streptokinase, EC 3.4.-; Tissue Plasminogen Activator, EC 3.4.21.68; Urinary Plasminogen Activator, EC 3.4.21.73