Serine protease inhibitor. Active against beta-trypsin and alpha-chymotrypsin with dissociation constants of 0.35 nM and 40 nM respectively. Inhibits factor XIa, but not other enzymes involved in coagulation and fibrinolysis. Does not inhibit subtilisin, lysyl endopeptidase, arginyl endopeptidase or papain. Inactive at pH 2.0, activity is restored after 10 minutes incubation at pH 8.0.