Chymotrypsin family serine proteases play essential roles in key biological and pathological processes and are frequently targets of drug discovery efforts. This large enzyme family is also among the most advanced model systems for detailed studies of enzyme mechanism and structure/function relationships. Productive interactions between these enzymes and their substrates are widely believed to mimic the "canonical" interactions between serine proteases and "standard" inhibitors observed in numerous protease-inhibitor complexes. To test this central hypothesis we have synthesized and characterized a series of peptide analogs, based on model substrates and inhibitors of trypsin, that contain unnatural main chains. These results call into question a long accepted theory regarding the interaction of chymotrypsin family serine proteases with substrates and suggest that the canonical interactions observed between these enzymes and standard inhibitors may represent nonproductive rather than productive, substrate-like interactions.