It has been shown that, in the organisms tested, the production of azurin or similar blue, copper-protein complexes is confined to bacterial species of the three genera Bordetella, Alcaligenes and Pseudomonas. In the strain of Pseudomonas aeruginosa used, there appeared to be no difference in the amount of azurin occurring when the strain was grown aerobically and anaerobically. The amount of azurin produced by representative strains of Ps. aeruginosa, B. bronchiseptica and A. denitrificans varied with the copper content of the medium. Above a level of 5 μg copper/ml of medium, the azurin content was constant for the three species tested; below a copper level of 0.5 μg/ml there was an almost total absence of azurin although good growth occurred. Under similar growth conditions, the azurin content of the three bacterial species studied was not significantly different.