The ability of three macrolide antibiotics (erythromycin, clarithromycin and azithromycin) to inhibit the expression of several pathogenicity traits of Pseudomonas aeruginosa at concentrations that do not affect the rate of growth of this microorganism was investigated. Sub-MICs of azithromycin manifested the broadest spectrum of action and strongly suppressed the synthesis of elastase, proteases, lecithinase and DNase. Clarithromycin and erythromycin were far less effective. Gelatinase was reduced almost to the same level by the three antibiotics, while haemolysins and lipase were only marginally affected. Loss of motility was a strain and drug-dependent event, but all the macrolides tested shared the ability to induce this effect. However, only azithromycin totally suppressed synthesis of pyocyanin in all isolates. These results indicate that newer macrolides and especially axithromycin are endowed with a remarkable ability to inhibit in vitro the expression of a number of physiological processes that are considered more essential than replication in the pathogenesis of P. aeruginosa. Since erythromycin sub-MICs have already been shown to exert beneficial effects in clinical practice, our data, pointing to a much higher potency of azithromycin, suggest its use in future studies.