1. Purified preparations of α2-macroglobulin inhibited the ability of urokinase to induce lysis of a fibrin plate, to activate plasminogen in a purified system, and to hydrolyse N-α-acetyl glycyl-l-lysine methyl ester. 2. Inhibition of urokinase by α2-macroglobulin was immediate: no progressive inhibition could be detected using fibrinolytic and esterolytic assays. Inhibition was competitive. 3. The fibrinolytic activity of euglobulin precipitate prepared from exercise plasma was also inhibited by α2-macroglobulin.