Ciprofloxacin was given perorally in two doses of 750 mg each with a 12-h interval starting 24 h prior to surgery, 400 mg of ciprofloxacin was given intravenously at the induction of anesthesia, and 400 mg of ciprofloxacin was given 12 h later to 21 patients undergoing elective colorectal surgery. The maximum concentrations in serum (mean value +/- standard deviation, 11.1 +/- 7.8 mg/liter) during surgery were reached 30 min after ciprofloxacin was administered. The ciprofloxacin concentrations in the intestinal mucosa samples were 2.7 to 37.8 mg/kg of tissue weight, and the concentrations in the fecal samples were less than 0.1 to 858 mg/kg. The aerobic and anaerobic bacteria in the fecal flora were suppressed markedly during the prophylaxis period. No postoperative infections occurred.