In recent years, various medical indwelling devices have been developed and used. Bacteria adhering to these devices often cause refractory infection. In the field of otolaryngology, refractory infection accompanying these medical indwelling devices such as middle ear ventilation tubes and artificial auditory ossicles has been reported. The concept of bacterial biofilm infection has been suggested as an explanation for the refractory infection. Furthermore it has been reported that a bacterial biofilm is involved in refractory infection unrelated to medical indwelling devices. Topical biofilm formation was detected in patients with chronic sinusitis, chronic purulent otitis media or habitual tonsillitis. In this study, we morphologically and quantitatively examined the biofilm-forming capacity of a clinically isolated strain of mucoid type Psecudomonas aeruginosa on Teflon to investigate the effects of macrolide antibiotics on bacterial biofilm formation. In the morphological examination, P.aeruginosa was cultured together with a Teflon sheet in minimal medium containing various concentrations of the macrolide antibiotics clarithromycin (CAM), erythromycin (EM) and midecamycin (MDM), at 37 degrees C for 7 days. The surfaces of the Teflon sheets were examined by electron microscopy. The adherent bacteria and biofilm formation on Teflon sheets soaked in minimal medium containing CAM or EM were found to be decreased in a dose-dependent manner. However, in the Teflon sheets soaked in minimal medium containing MDM, there was no decrease in biofilm formation regardless of the MDM concentration. In the quantitative examination, P. aeruginosa was cultured in minimal medium containing various concentrations of the macrolide antibiotics at 37 degrees C for 7 days together with Teflon beads. The levels of hexose, protein and alginate adhering to the Telfon beads were quantified as an estimation of biofilm formation. On Teflon beads treated with CAM or EM, there were dose-dependent decreases in hexose, protein and alginate levels. In particular, marked decreases were noted when CAM and EM concentrations were 10 micrograms/ml or more. Furthermore, there was no significant difference between CAM and EM. However, in the presence of MDM, there was no decrease in hexose, protein or alginate levels regardless of the MDM concentration. The minimal inhibitory concentration (MIC100) of each macrolide against P. aeruginosa used in this experiment was 100 micrograms/ml or more. There may be no bactericidal effect on this strain at the macrolide concentrations used in this experiment. However, this experiment used 7-day treatment. The long-term bactericidal activity of macrolides was examined. In the presence of CAM or MDM, bacterial levels after culture were similar to preculture levels or slightly lower than the preculture levels. In the presence of EM, bacterial levels were similar to the preculture levels. These results demonstrated that CAM and EM, which are 14-membered macrolides inhibited biofilm formation, while MDM which is 16-membered macrolide, did not. These inhibitory effects of CAM and EM may be related to actions other than bactericidal activity. In our experiment, CAM and EM inhibited biofilm formation at 10 micrograms/ml or more. This concentration corresponded to 1/20 x MIC. This concentration can be achieved in tissues, nasal discharge and sputum with actual clinical doses. Therefore, these agents may be effective against biofilm disease caused by P. aeruginosa in the field of otolaryngology.