Tissue-type plasminogen activator (t-PA), purified from the culture fluid of a stable human melanoma cell line, is a serine protease, different from urokinase, with a molecular weight of about 70,000. It is composed of one polypeptide chain, which is converted to a two-chain molecule by limited plasmic action. Activation of plasminogen to plasmin occurs by cleavage of the Arg 560-Val 561 peptide bond. Kinetic analysis has shown that the activation obeys Michaelis-Menten kinetics and that the presence of fibrin strikingly enhances the activation rate by increasing the affinity of plasminogen for fibrin-bound t-PA. The directed action of plasmin toward fibrin in vivo, might be explained by the low Michaelis constant in the presence of fibrin (0.16 microM), which allows efficient plasminogen activation on a fibrin clot, while its high value in the absence of fibrin (65 microM) prevents efficient activation in plasma. Plasmin formed on the fibrin surface would then be protected from rapid inactivation by alpha 2-antiplasmin. An important consequence of this molecular model for physiological fibrinolysis is that specific thrombolysis is only expected with the use of a specific plasminogen activator, which confines activation to the fibrin surface. Studies on the thrombolytic properties of purified t-PA in various animal species and in humans have revealed a higher specific thrombolytic activity than urokinase. Thrombolysis could be achieved without causing significant plasminogen activation, alpha 2-antiplasmin consumption, or fibrinogen breakdown. Alternatively, pro-urokinase, the zymogen precursor of urokinase, also displays a certain degree of fibrin specificity. Its mechanism of action and potential therapeutic value remain to be established.