This paper describes the development of several competitive inhibitors and mechanism-based inactivators of subtilisin BPN'. Various methyl arylalkanesulfonates were prepared and shown to be either competitive inhibitors or selective methylating reagents for the epsilon-2-N of the active-site His. Styrene sulfonyl chloride was shown to be a good covalent inactivator and benzyl N-(N-Boc-L-phenylalanyl)-L-aziridine-2-carboxylate was a strong competitive inhibitor. The second-order rate constant of inactivation and the K(i) of the enzyme reacting with the methylating reagents and other inactivators were determined. The methylated enzyme was purified to homogeneity and the kinetic constants for the enzyme-catalyzed ester and amide hydrolyses were determined. It was established that the methylated enzyme lost most of the amidase activity while the esterase activity was still significant and useful for peptide synthesis via aminolysis. A mechanism involving ring flipping of the active-site imidazole, first proposed for methylchymotrypsin activity, was also proposed to explain methylsubtilisin-catalyzed reactions.