We describe a reverse-phase high-pressure liquid chromatography method for the quantitation of a new quinoline carboxylic acid antimicrobial agent, ciprofloxacin (Bay o 9867). This assay utilizes the intrinsic fluorescence of ciprofloxacin for primary detection but employs UV absorption as a secondary detection system. Mobile phases contained methanol and phosphate buffer and used a common C18 mu Bondapak column. A single precipitation step of a 50-microliter specimen was the only sample preparation necessary. The assay is linear from 2,000 to 10 ng/ml and sensitive to 5 ng/ml. The mean recovery of ciprofloxacin from serum was 105.7%. The coefficient of variation was less than or equal to 3.1% for same-day precision and less than or equal to 6.3% for assay-to-assay precision. Because the assay requires only small specimen volumes and minimal sample preparation and because of its defined characteristics, this assay would be ideal for clinical trials and pharmacokinetics studies of ciprofloxacin.