Pseudomonas aeruginosa exotoxin A has been shown to catalyze the transfer of the adenosine 5'-diphosphate (ADP)-ribose moiety of nicotinamide adenine dinucleotide onto elongation factor 2, resulting in the inhibition of mammalian protein synthesis. The enzymatic activity (ADP-ribosyl [ADPR]-transferase) is thought to account for the toxicity of exotoxin A. The distribution of the expression of exotoxin A within Pseudomonas species was examined. Laboratory strains as well as clinical isolates of Pseudomonas aeruginosa were tested. The production of exotoxin A was determined by assaying for ADPR-transferase activity in dialyzed frozen (-20 degrees C) and thawed cell-free supernatants from 22-h cultures or in 10-fold-concentrated supernatants. In addition, toxin production was detected immunologically using a modified Elek test. Exotoxin A production was detected in approximately 90% of the 111 isolates of P. aeruginosa. In contrast, none of the other species of Pseudomonas examined produced exotoxin A detectable by either ADPR-transferase activity or immunological reactivity.