To estimate the efficacy of ciprofloxacin in the treatment of pelvic peritonitis, a pharmacokinetic study was conducted in four Japanese subjects. Ciprofloxacin was administered intravenously at a dose of 300mg for 1h to patients with pelvic peritonitis. Ascites was collected by culdocentesis. The concentrations of ciprofloxacin in blood and ascites were measured by a bioassay, using Escherichia coli Kp as the test organism and heart infusion agar as the medium. The ciprofloxacin concentration in ascites ranged from 3.01 to 9.41µg/ml. The values for the arithmetic mean of ascites/serum ranged from 4.01 to 19.37, which showed that the penetration of ciprofloxacin was higher in ascites than in serum. The concentrations of ciprofloxacin are generally higher in ascites than in blood. Taking the antimicrobial spectrum of ciprofloxacin and the causative organisms of pelvic peritonitis into account, intravenous ciprofloxacin could be a good candidate for use in the treatment of pelvic peritonitis from the point of view of pharmacokinetics.