Protease inhibitor with activity against cysteine, aspartic and serine proteases. Highest activity against serine proteases, in particular trypsin and trypsin-like proteases. Stable at acidic pHs. Inhibitory activity is unaffected after 1 hour at pH 3.0 and 96 degrees Celsius. Inactivated after 1 hour at pH 10.7 and 96 degrees Celsius. Occurs as a monomer, dimer or trimer. The dimer may be the active form. Binds calcium, probably through His-3 to His-6. Belongs to the Bowman-Birk serine protease inhibitor family.