Gentamicin and tobramycin inhibited the accumulation of extracellular proteases secreted by Pseudomonas aeruginosa. The secretion of protease was inhibited at concentrations of these drugs that were below the level required to inhibit general protein synthesis. Neither magnesium ions nor high ionic strength antagonized the ability of the aminoglycosides to block secretion of the proteases. Under these culture conditions magnesium ions were shown to antagonize the effects of the aminoglycosides on protein synthesis and aminoglycoside-mediated lysozyme lysis of P. aeruginosa. These results suggested that the drugs blocked secretion of the proteases by acting at the level of the outer membrane.