Pulmonary infection with Pseudomonas aeruginosa is a major cause of morbidity and mortality in cystic fibrosis (CF) patients. P. aeruginosa toxin is one of several proposed virulence factors which may be responsible for chronic P. aeruginosa infections in these patients. With a highly specific, sensitive, and quantitative radioimmunoassay (RIA) and a cell culture assay, the humoral immune responses of CF patients in terms of total antitoxin, antitoxin immunoglobulins A and M, and neutralizing antitoxin were compared with those of P. aeruginosa-infected intensive care unit patients and controls. The P. aeruginosa-infected CF patients were divided into severe and moderate disease groups based on mortality observed over an 8-year period. The intensive care unit patients were divided by the site of infection and the controls were healthy children and uninfected CF patients. Antibodies to toxin were found in the sera of all subjects by radioimmunoassay. Neutralizing antibody was associated with current infection. Elevated titers of antitoxin immunoglobulin A were found only in subjects with pulmonary P. aeruginosa infections. No significant differences in any antibody class were observed between the severe and moderate disease groups. In addition, no differences were observed in the antitoxin immune response of chronically infected CF patients and intensive care unit patients with acute pulmonary infections.