The fast-acting and physiologically most important inhibitor of plasmin in human plasma is a recently discovered and purified alpha 2-glycoprotein with a molecular weight of 65,000-70,000 daltons occurring at a concentration of 1 muM. The inhibitor rapidly forms a completely inactive 1:1 stoichometric complex with plasmin through reaction with the B chain (light chain) of the enzyme, which contains the active center. It also reacts with trypsin and very slowly with urokinase and with some other enzymes in purified systems, but its role in vivo as an inhibitor of proteases other than plasmin seems negligible. Antiplasmin is the only plasma protein that can inhibit the fibrinolysis associated with transformed or malignant cells. The plasmin-antiplasmin complex contains neoantigenic structures not present in the parent molecules that may form the basis of immunochemical methods for detecting activation of the fibrinolvtic system in blood.