The safety, tolerance, and influence on the faecal flora of volunteers of fleroxacin was studied following a daily dose of 800 mg for seven days. A major reduction was seen in the numbers of Escherichia coli, but there was no change in anaerobic bacteria or the faecal streptococci. Ophthalmological examinations and laboratory evaluations revealed no untoward effect. Reduced appetite and flatulence were observed in one volunteer each, two reported loose stools, and five complained of sleep disturbances. All volunteers completed the study.