Analogs of the epoxysuccinyl peptide cysteine proteinase inhibitor, EP-475 (2a), in which the free carboxylate has been replaced by hydroxamic acid, amide, methyl ketone, hydroxyl, and ethyl ester functionalities, have been synthesized. Individual rate constants of inhibition of papain were determined for these inhibitors. The results show that a carbonyl-containing functionality is necessary for good activity. The pH dependence of the inhibition of papain was determined for a nonionizable EP-475 (2a) analog; inhibition was found to depend on two acidic ionizations (pKas of 3.93 and 4.09) of papain. Implications for the mechanism of action of epoxysuccinyl peptides with papain are discussed.