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Evidence is presented that Zn(2+) and certain other multivalent cations inhibit the uptake and oxidation of substrates by Pseudomonas aeruginosa and by other bacterial cells. Tris(hydroxymethyl)aminomethane augmented this effect. Inhibition by these multivalent cations could be prevented by incubation of cellular suspensions of P. aeruginosa in phosphate buffer. In the case of other microorganisms, incubation in the presence of ethylenediaminetetraacetate also prevented the inhibition. Inhibition by Zn(2+) of the uptake and of the oxidation of substrates could be reversed by the addition of Mg(2+). Zn(2+) was shown to inhibit noncompetitively the uptake of the three substrates studied, l-alanine, d-glucose, and citrate.