Methods of cell culture which enable the control of specific growth rate and expression of iron-regulated membrane proteins within Gram-negative biofilms were employed for various clinical isolates of Pseudomonas aeruginosa taken from the sputum of cystic fibrosis patients and of a laboratory strain of Escherichia coli. Susceptibility towards ciprofloxacin was assessed as a function of growth-rate for intact biofilms, cells resuspended from the biofilms and also for newly formed daughter cells shed from the biofilm during its growth and development. Patterns of susceptibility with growth rate were compared to those of suspended cultures grown in a chemostat. In all instances the susceptibility of chemostat cultures was directly related to growth rate. Whilst little difference was observed in the susceptibility pattern for P. aeruginosa strains with different observed levels of mucoidness, such populations were generally more susceptible towards ciprofloxacin than those of E. coli. At fast rates of growth P. aeruginosa cells resuspended from biofilms were significantly more resistant than chemostat grown cells. Intact P. aeruginosa biofilms were significantly more resistant than cells resuspended from them. This is in contrast to E. coli, where cells resuspended from biofilm and intact biofilms were, at the slower rates of growth, equivalent and significantly more susceptible than chemostat-grown cells. At high growth rates all methods of E. coli culture produced cells of equivalent susceptibility. For all strains, daughter cells dislodged from the biofilms demonstrated a high level of susceptibility towards ciprofloxacin which was unaffected by growth rate. This sensitivity corresponded to that of the fastest grown cells in the chemostat.