Various thrombolytic agents have been studied as activators of the plasminogen-plasmin system for thrombolysis of thrombus formation. They include streptokinase, urokinase, tissue plasminogen activators, single-chain urokinase plasminogen activator, and anisoylated or acylated plasminogen-streptokinase activator complex (APSAC), only some of which are commercially available. All thrombolytic agents, including APSAC (not commercially available), recombinant tissue plasminogen activator, and prourokinase, generate great quantities of degradation products of fibrinogen or fibrin. All of the second-generation thrombolytic agents induce systemic activation of the entire fibrinolytic system, and none are capable of specifically activating the fibrinolytic system at the site of thrombus formation. The most systemically active agent known at the present time is APSAC. Trials show that bleeding occurs as frequently with the second-generation agents as with the older agents, and further studies may even find that the newer agents are associated with more bleeding than urokinase and streptokinase have been. With knowledge of the properties of the various thrombolytic agents available today, the physician can intelligently select the optimal agent for a given patient problem.