Several mucoid strains and one non-mucoid strain of Pseudomonas aeruginosa were grown in subinhibitory concentrations of various aminoglycosides, beta-lactams, and ciprofloxacin. At antibiotic concentrations that did not affect bacterial growth, aminoglycosides inhibited the excretion of alginate capsule and the production of the iron-chelating siderophores. The same concentrations also disturbed other aspects of the bacterium's iron metabolism, manifested by reduced cellular levels of cytochromes and catalase. beta-lactams, except for cefotaxime, and ciprofloxacin did not reduce alginate excretion. These effects on virulence factors are likely to contribute to the efficacy of aminoglycosides.