A method for the study of membrane associated biofilm populations of Escherichia coli ATCC 8739, which utilised modified Swinnex filter units was developed. Biofilms were established under carbon limitation in a chemically defined simple salts medium. Cells, pressure filtered onto cellulose nitrate membranes in situ, were perfused from the sterile side. Steady state conditions were attained at which the growth rate of the sessile cells could be demonstrated to be proportional to the flow rate of medium. The antibiotic susceptibility of these biofilms was examined by including ciprofloxacin within the perfusing medium. Susceptibility of the biofilms to ciprofloxacin was found to be affected, not only by its concentration, but also to the mode of exposure and growth rate. Postantibiotic effects induced by ciprofloxacin were demonstrated in batch and chemostat cultures of E. coli, as well as in biofilms. Such effects were dependent upon the total antibiotic exposure and the growth rate of the biofilm. Filamentation of cells of E. coli was found to occur, on exposure to ciprofloxacin. This phenomenon was seen in biofilm associated cells, those shed from the biofilms and also in batch cultures. The occurrence and length of the filamented cells corresponded to the changes in steady state observed for the biofilm after exposure to ciprofloxacin. Clinical pharmacokinetic profiles for both infused and oral doses of ciprofloxacin were modelled successfully on biofilms of E. coli, From this it was concluded that oral dosing was preferable to infusion for the suppression of growth of biofilms of E. coli.