Most strains of Pseudomonas aeruginosa produce three proteases with broad substrate specificities. One of these enzymes has elastolytic activity (P. aeruginosa elastase). This elastase has tissue-damaging activity and is capable of degrading various plasma proteins such as immunoglobulins, coagulation and complement factors, and alpha-proteinase inhibitor. There is evidence for a role of elastase in localized infections such as experimental pseudomonas keratitis, pneumonia, and burn infection. Once colonization and invasion has occurred and septicemia has been established, these enzymes are probably less important. Elastase is probably best classified as a virulence-enhancing factor in certain types of infections.