Aprotinin acts as a reversible serine protease blocking and a binding agent. It is widely employed as an inhibitor of trypsin. It is also used as a proteolytic inhibitor in radioimmunoassays of polypeptide hormones. It is involved in the purification of urokinase, trypsin, and chymotrypsin on immobilized aprotinin. Further, it is used in the quantification of kallikrein activity in mixtures of esterases and proteases.