It is possible to analyze corium collagen from leather by routine methods for hydrolysis and amino acid analysis. Deteriorated leather may be characterized by containing large amounts of ammonia and by an altered amino acid distribution compared to that of new leather. It is shown that proline and lysine are the two amino acids mostly affected by deterioration. These two amino acids together with isoleucine may, irrespective of the state of deterioration, provide a statistical method to establish the animal origin of the leather. Attempts with accelerated ageing methods lead to results similar to those found for historical leather.