AbstractDerivatives of the thiirancarboxylic acid building‐block containing a peptide bond were synthesised and screened against the model cysteine protease papain. The most active of the series showed a second‐order rate constant of inactivation comparable to that of the parent compound. The insertion of a peptide moiety seems to compensate the lack of a free carboxylate interacting with the histidinium ion at the enzyme's active site.