The postantibiotic effects of subinhibitory concentrations (PA SMEs) and virulence factor alterations induced by ciprofloxacin, tobramycin and netilmicin in Pseudomonas aeruginosa were studied. After induction of the postantibiotic phase (PA) (2x or 4x MIC) the cultures were exposed to subinhibitory concentrations (0.1, 0.2 and 0.3x MIC) of the same antibiotic (PA SME). The regrowth of treated as well as control cultures was followed for 24 or 45 h. In the sterile culture filtrates obtained from these bacterial cultures, elastase and proteinase were determined. Ciprofloxacin and aminoglycosides exhibited PA SMEs of 35-35 h for certain combinations of supra-subinhibitory antibiotic concentrations. Longer PA SMEs were observed after treatment with higher sub-MICs. Tobramycin at 0.2 and 0.3x MIC (postantibiotic phase induced by 2x MIC) and at alt sub-MICs added to the bacteria previously exposed to 4x MIC do not allow any regrowth of bacterial culture. PA SMEs of tested antibiotics affected virulence factors of P. aeruginosa. Elastase compared to proteinase was suppressed more effectively. Ciprofloxacin at 0.3x MIC reduced elastase and proteinase activity most significantly (to 14.2 and 60% of the control values).