In continuous-flow culture, long generation times and high bacterial counts favour survival of bacteria. A chemotherapeutic agent that achieves a bactericidal effect under these circumstances can therefore be seen as highly effective. In our continuous-flow culture we obtained bactericidal effects with ciprofloxacin 1-2 mg/L, cefotaxime 4 mg/L and mezlocillin 32 mg/L. These effects were seen irrespective of whether conditions were aerobic or anaerobic. There were no significant differences between monocultures and mixed cultures simulating faecal flora with the various Escherichia coli strains tested. Cefotaxime had an initial effect but an increase in counts was then observed as a result of regrowth of E. coli survivor strains in aerobic monoculture and mixed cultures. Mezlocillin was completely bactericidal in monocultures, but regrowth occurred in mixed cultures under anaerobic conditions. Neither the bacterial composition of this culture nor the resistance pattern explained this regrowth. These results were observed in long-term experiments followed for up to 7 days. We conclude that the antibiotics tested are highly effective against E. coli under unfavourable conditions simulating in-vivo situations.