peptide thioneamides are provided as synthetic substrates for cysteine proteases . the compounds of the invention are peptides consisting of one or more blocked or unblocked amino acids wherein the terminal carboxy of the peptides are thionated and forms a thioneamide linkage with a fluorogenic or chromogenic leaving group . the alpha conbonyls of the remaining amino acids are thionated or unthionated . the invention includes methods and kits for using the peptide thioneamides .