Toxin A is a protein with a molecular mass of approximately 66,000 daltons whose production is regulated by iron. The toxicity of toxin A is due to its ability to inhibit protein synthesis. A role for toxin A in infections due to Pseudomonas aeruginosa is supported by its extreme toxicity, its production by most clinical isolates, the decreased virulence of nontoxic mutants, and the results of immunologic studies of animals and humans.