Inhibits subtilisin-type microbial serine proteases including proteinase K, subtilisin BPN', subtilisin Carlsberg, subtilisin E, A.oryzae protease and S.griseus alkaline protease. Weakly inhibits pronase E. Does not inhibit trypsin or chymotrypsin. Belongs to the protease inhibitor I13 (potato type I serine protease inhibitor) family.