The suppression of alimentary canal flora by the three quinolones nalidixic acid, ciprofloxacin and pefloxacin was investigated in fifteen volunteers. They received the three quinolone compounds in tablet form both uncoated and colon-coated. Escherichia coli suppression was poor under nalidixic acid, but complete under ciprofloxacin and pefloxacin for both administration forms. The indigenous anaerobic flora contributing to the control of aerobic Streptococcus faecalis and Candida albicans in the intestines ('colonization resistance') was not affected by nalidixic acid and pefloxacin, and only slightly by ciprofloxacin. Out of the three quinolone compounds, only colon-coated pefloxacin was associated with a considerable absorption rate at colonic level. Using these criteria of successful Escherichia coli clearing from the intestinal canal--left the indigenous flora more or less intact (in a 'selective' way)--and a good absorption rate, pefloxacin is found to be superior to ciprofloxacin and nalidixic acid. These results suggest that a colon-coated tablet with a low dose of pefloxacin is a promising administration form in the therapy of recurrent urinary tract infections and diarrhoeal diseases and in the prevention of gut colonization in immunocompromised hosts.