Most Pseudomonas aeruginosa strains produce exotoxin A and two extracellular proteases (elastase and alkaline protease). Exotoxin A is a lethal toxin that inhibits protein synthesis in mammalian cells by the same mechanism as diphtheria toxin. It is generated in clinical and experimental animal infections. Passive or active immunization against this toxin gives significant protection against experimental infections with exotoxin-producing strains. The proteases have tissue-damaging activity and are capable of degrading various plasma proteins such as complement and coagulation factors. Proteases probably play a part in localized pseudomonas infections such as keratitis, pneumonia and burn infection. When invasion and colonization have occurred and septicemia is established, these enzymes probably are less important.