Fibrinolysis inhibition by a mixture of potato inhibitor and E-aminocaproic acid was greater than might be expected from the sum of the antifibrinolytic effects of these inhibitors investigated separately. This inhibition was observed in studies on the plasma euglobulin fraction and in a system containing isolated elements of the fibrinolytic system. The synergistic antifibrinolytic action of the potato protease in hibitor and E-aminocaproic acid is probably due to the fact that these inhibitors have different mechanisms of action and thus there is no competition between them for the effectors in the enzyme molecule.