A series of peptidyl N-nitrosoanilines were designed, synthesized, and evaluated as inactivators of cysteine protease papain and serine protease chymotrypsin. These new compounds exhibited different inhibitory activities toward the cysteine protease papain in a time-and concentration-dependent manner with second-order rate constants (k(inact)/K-I) ranging over 2 orders of magnitude from 0.604 M-1 s(-1) (1) to 100.36 M-1 s(-1) (7). No inactivation was observed for serine protease chymotrypsin. Formation of the S-NO bond in papain is supported by several lines of evidences from both spectroscopic studies and chemical analyses. The pH profile study on inactivation of papain by compound 1 was conducted over the pH range 3.2-9.2 to provide more insight into the mechanism of the inactivation process. The covalent yet recoverable cysteine protease inactivation process offers mechanistic implications and endows this new family of inactivators with special properties that are suitable for the development of stable and potent cysteine protease inhibitors.