An investigation of the salt requirements for pyocyanin production by Pseudomonas aeruginosa has been carried out, and the mineral balance for maximum pyocyanin production determined. By employing a medium containing the required salt balance, it has been possible to obtain a fourfold increase in pyocyanin production over that obtained on other media usually employed for that purpose.The ions, Mg, SO4, K, PO4, and Fe have been found essential to the formation of the pigment. Optimum production of pyocyanin was obtained by the use of 0.04% dipotassium hydrogen phosphate, 2.0% magnesium sulphate septahydrate, and 0.001% ferrous sulphate septahydrate in a medium containing glycerol, glycine, and leucine. In the presence of minimum amounts of Mg and SO4, maximum pyocyanin production can be obtained by the addition of increased amounts of either Mg or SO4.