An exotoxin, toxic to both mice and cultured cells, was isolated from cultures of Pseudomonas aeruginosa. Relatively small amounts of the exotoxin inhibited the uptake of uridine and amino acids by Vero cells. Within limits, this toxic action was reversible and could be inactivated by heating at 70 C or by proteolytic digestion, but it was not affected by nucleases. The inhibitory activity of the toxin could also be decreased by rabbit antiserum. Metabolic activities of Vero cells, generally associated with the production of energy, did not appear to be affected by the toxin.