Pseudomonas aeruginosa is an opportunistic pathogen whose adaptability, ubiquitousness, and pathogenicity are closely related. Both cell-associated and extracellular products of P. aeruginosa contribute to its virulence. Surface structures, including pili and the polysaccharide capsule or glycocalyx, appear to mediate the initial attachment of P. aeruginosa to its prospective host, thus permitting colonization. Extracellular enzymes such as alkaline protease, elastase, phospholipase C, and exotoxin A degrade infected tissues and promote bacterial invasion. When dissemination occurs, systemic disease results, often with fatal consequences. Although extracellular enzymes of P. aeruginosa figure prominently in local disease processes, exotoxin A and endotoxin are primarily responsible for systemic disease. The most protective antibodies presently known are directed toward the nontoxic portions of P. aeruginosa lipopolysaccharides that serve no known virulence function per se. However, there is preliminary evidence that the protective activity of these opsonic antibodies may be augmented by toxin-neutralizing antibodies directed toward the lipid A moiety of endotoxin and exotoxin A.