The definition of N-terminal groupings can be a tool for describing the hydrolitic degradation of collagen in leather. The reaction to the phenyl isothiocyanate (PITC) on the N-terminal extremities of the polypeptide chains leads to the formation of phenylthiohydantoin derivatives (PTH) which differ according to the particular amino acid. Their separation by high-performance liquid chromatography with inverted phase polarity is carried out with a mobule phase concentration gradient in 15 min on a C₁₈ column. Their identification by means of calibrations allows us to evaluate the different N-terminal amino acids of the protein at a given instant and also to determine by comparison the progression of the cuts in the polypeptide chains. Particular attention is given to preparing the leather for analysis. Experiments have been made on artificially aged leathers and on old leathers. A methodology is proposed. -- CAL