This inhibitor has two domains, each with separate antiprotease activity. Inhibits bovine trypsin and chymotrypsin, in a molar ratio of 1:1. The trypsin inhibition of FBI is independent of chymotrypsin inhibition, but the chymotrypsin inhibition is not completely independent of trypsin inhibition. Belongs to the Bowman-Birk serine protease inhibitor family.