Pseudomonas aeruginosa is a microorganism of the terrestrial and aquatic environment which may cause pulmonary, urinary and corneal infections. The pathogenesis of Pseudomonas-induced diseases seems to be linked to the production of extracellular substances such as exotoxins, hemolysins, leukocidins and hydrolytic enzymes, including various peptidases. Anyway there is not a clear view on the role other proteases have in the mechanism of pathogenesis, and even if their activity may always be associated to the toxicity of the microorganism. We have therefore determined the activity of a number of eso- and endopeptidases in 17 strains of P. aeruginosa isolated from patients with urinary pathologies. Four of these peptidase activities, namely elastase, neutral protease, aminopeptidase I and leucine aminopeptidase show a positive correlation with three parameters selected as indices of toxicity, i.e. the mucoid appearance, the gentamicin resistance and the adhesivity of colonies.