The colonic microflora was studied before, on the fifth day of dosing, and 2 weeks after a 5-day oral course of ciprofloxacin 500 mg BID given to 7 young and 7 elderly, healthy volunteers. Considerable changes in the aerobic microflora were found, while the effects on the anaerobic bacteria were less pronounced. Despite larger absolute bioavailability of the first dose in the elderly (77 vs. 63%; p less than 0.05), the effect of ciprofloxacin on the microflora was similar in the two groups of volunteers. A higher number of clostridia were detected in faeces from elderly subjects before, during, and after the ciprofloxacin regimen.