Levels of antibody to toxin A of Pseudomonas aeruginosa were determined by a solid-phase radioimmunoassay. Mean (+/- SEM) peak levels of IgG in 24 normal soldiers were 2.6 +/- 0.5 microgram/ml, white mean peak levels in 12 patients colonized with and 13 patients infected in sites other than the blood with toxin A-producing strains were 16.7 +/- 7.0 and 17.1 +/- 4.4 microgram/ml, respectively. Levels of IgG were determined in 52 patients with pseudomonas bacteremia, and those surviving and those dying of bacteremia due to toxin A-producing strains had mean peak levels of 25.8 +/- 5.5 and 4.6 +/- 2.0 microgram/ml, respectively. The antitoxin response in sequential bacteremic sera began shortly after onset of bacteremia and decreased gradually, but antitoxin could be recalled promptly upon reinfection with Pseudomonas. Death from pseudomonas bacteremia was significantly associated with infection by a toxin A-producing strain, presence of underlying disease, hypotension, and antitoxin level of < 2 microgram/ml.