The bactericidal activity of OPB-2045 (1-(3,4-dichlorobenzyl)-5-octylbiguanide monohydrochloride hemihydrate) at several concentrations against Pseudomonas aeruginosa IFO 13275 was investigated morphologically by transmission and scanning electron microscopy. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of OPB-2045 against P. aeruginosa were the same, at 12.5 microg mL(-1), suggesting that it may be a suitable disinfectant for use in the medical field. Test bacteria were treated at concentrations of one half the MIC value (6.25 microg mL(-1)), the MIC value (12.5 microg mL(-1)), twice the MIC value (25 microg mL(-1)) or ten times the MIC value (125 microg mL(-1)) at 37 degrees C for 30 min or 6 h and the cells were then examined by transmission and scanning electron microscopy. The cell damage evident after 6h incubation was greater than observed after 30 min incubation. Especially, at one half the MIC, no cell damage was evident after 30 min incubation, but damaged cells were observed after 6 h incubation. The proportion of empty cells of P. aeruginosa increased as the concentration of added disinfectant was increased, and the release of intracellular components was also recognized. These results suggest that OPB-2045 acts on the cell membrane and cell wall of P. aeruginosa, and destroys their integrity at the level of the MIC (MBC). With the increase in OPB-2045 concentration and the increase in reaction time, the bactericidal effect increased markedly. Agglutination of the cells was observed at high concentrations of OPB-2045. This indicates that the bactericidal effect at high concentrations of OPB-2045 differs from that at low concentrations. A clear cell-damaging effect against the test strain was recognized which was dependent on the OPB-2045 concentration and the incubation time. From experiments concerning the relationship between the number of surviving bacteria and MIC values in soybean casein digest broth, the decrease in bacterial numbers was found to be dependent on the OPB-2045 concentration. We conclude that it would be a useful contribution to the medical field to supply a new disinfectant to be employed in preventive countermeasures against infection caused by pathogenic bacteria.