The postantibiotic effect (PAE) has been classically defined as the suppression of bacterial growth that persists after limited exposure of organisms to antimicrobial agents. Morphology and haemolysin activity during the PAE of three quinolones on Escherichia coli were examined in this study. A one hour exposure to the quinolones, CI-960, enoxacin and ciprofloxacin, produced a PAE of 0.5-2.0 h. When determinated by Coulter counter, at 0.5 x MIC of enoxacin or CI-960 after 1 h exposure, 58% or 42% cells, respectively, of the treated cells were filamentous (cell length greater than 12 microns). After drug removal, the population of the filamentous cells decreased, however, after even 4 h, 12% and 2% of the cells were still filamentous after exposure to enoxacin or CI-960. Further morphological studies during the PAE showed that the first division of the filamentous cell was asymmetrical, and both bacterial cell division and septation were delayed after exposure to 0.5 MIC of CI-960. Following quinolone removal, the treated E. coli did not exhibit normal activity of haemolysin for at least 2 h. Internal haemolysin activity was adversely affected for 1 h. The results of this study suggest that any consideration of postantibiotic effects should include the residual antibiotic effects on bacterial morphology and virulence factors, in addition to the defined suppression of bacterial regrowth.