Factors that influence production of exotoxin A by a nonproteolytic strain of Pseudomonas aeruginosa (PA-103) were studied. Combinations of amino acids and nucleotides were not able to produce good growth and production of toxin. The organism appeared to require some factors in the dialysate of complex media for good growth and maximal production of toxin. Pure proteins, such as crystalline bovine albumin, enhanced growth as well as production of toxin. Aeration of the culture, the presence of glycerol as a carbon source, and an incubation temperature of around 32 C appeared to be essential for maximal production of toxin. Nucleic acids of various types, particularly after autoclaving, inhibited production of toxin but enhanced growth of the organism. There are some heat-labile, cell-bound toxins that can be liberated from live cells with sonic vibration. These toxins account for 15%–20% of the toxicity of the whole culture, but they appeared to be serologically unrelated to the exotoxin.