Direct evidence is presented for the production of an exotoxin by Pseudomonas aeruginosa multiplying at the burned site in an infected mouse. Pseudomonas toxin was assayed by measurement of its ability to catalyze the transfer of radioactivity from [14C]adenine-labeled nicotinamide adenine dinucleotide to elongation factor 2 (adenosine diphosphate ribosylation activity). This enzyme activity was found in saline extracts of burned infected skin but was not present in similar extracts of burned uninfected skin. It was detected in the serum of infected animals by 26 hr after infection. The level of active elongation factor 2 in the livers of infected mice was reduced significantly after infection. These data suggest that pseudomonas exotoxin, produced by bacteria multiplying at the burn site, enters the circulation and is disseminated to different organs where it acts by depletion of elongation factor 2 and thus causes a reduction in protein synthesis.