Elastolysis of insoluble elastin by Pseudomonas aeruginosa elastase was found to be less specific (higher apparent Km value) but more active (higher activity) than with pancreatic elastase. Furthermore, pancreatic and P. aeruginosa elastases act synergistically during the initial stages of elastolysis. After extensive hydrolysis, the size distribution of digestion products was lower with P. aeruginosa than with pancreatic elastase. The higher extent of hydrolysis may be explained by the fact that, if pancreatic elastase needs at least six sub-sites for activity, P. aeruginosa elastase may hydrolyse tetrapeptides such as tetraalanine, or synthetic substrates such as furylacryloyltripeptides FA-X-Leu-Y, X and Y being Gly and/or Ala.