The fibrinolytic system is activated by conversion of the enzymatically inactive precursor, plasminogen (profibrinolysin) to the active enzyme plasmin (fibrino-lysin). A variety of substances are capable of activating plasminogen; depending on their mode of action they are either direct or indirect activators. In analogy, also inhibition follows a direct and an indirect pathway (affecting plasmin or activation of plasminogen, respectively). The physical chemical parameters and molecular characteristics of plasminogen and plasmin are discussed, with special reference to the activation process. Among the activators, urokinase, tissue activators, blood activator and streptokinase are mentioned in detail. The de- scription includes the presently available information on physical chemical data, on the activation mechanism and on the biological function. Particular attention is drawn to the activation with strepto-Mnase. The large number of inhibitors is represented mainly by [epsilon]-aminocaproic acid and the plasma inhibitors. In a final section the mechanism of fibrin digestion by plasmin is discussed and an attempt is made to demonstrate the relationship between the fibrinolytic system and other processes of the organism.