The yields of exotoxin A in Pseudomonas aeruginosa cultures were influenced by the concentration of iron in the culture media. When the iron concentration of the culture media was increased from 0.05 to 1.5 microgram/ml, there was at least a 90% decrease in exotoxin A (measured both by enzymatic activity and by mouse lethality) and a slight increase in the growth of the bacteria. The addition of iron as late as 13 h after initiation of growth repressed further measurable increases of exotoxin A within 3 h. Intracellular toxin levels were also reduced by increasing the iron concentrations of the culture media. The addition of 3.0 microgram of iron per ml did not significantly alter either the enzyme activity of preformed crude or purified exotoxin A or the mouse toxicity of the pure toxin. Thus it appears that either the rate of production or the rate of intracellular degradation of exotoxin A is regulated by the concentration of iron in the culture media.