The quinolones exhibit a selective suppressive effect on the intestinal microflora. The aerobic Gram-negative bacteria are strongly suppressed, while the aerobic Gram-positive bacteria are less affected, with ciprofloxacin and ofloxacin having the greatest effect. The anaerobic microflora is not affected by administration of norfloxacin, but is suppressed slightly by ciprofloxacin and ofloxacin, and moderately by sparfloxacin and temafloxacin. Very high concentrations of the quinolones are obtained in faeces, far exceeding the minimum inhibitory concentration for most aerobic and anaerobic bacteria. The discrepancy between in vivo and in vitro outcome is explained by the binding of the quinolones to faeces, and by inoculum effects. These ecological properties of the quinolones on the intestinal microflora make them suitable for treatment of bacterial enteric infections, selective decontamination and prophylaxis against travellers' diarrhoea.