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To investigate the possibility of therapeutic use of ciprofloxacin in infections of the biliary tract, the serum and bile kinetics, and the biliary, urinary and fecal elimination of this new broad-spectrum quinolone were studied in 12 recently cholecystectomized patients with T-tube drainage. Ciprofloxacin concentrations were determined by simultaneously performed HPLC and microbiological assay in serum, urine, and bile over a 24-hour period following oral administration of a single dose of 500 mg of the drug. The two methods yielded similar values both in the serum and in the urine. Average peak serum concentrations were 0.97 +/- SEM 0.17 microgram/ml (HPLC) and 1.08 +/- 0.19 microgram/ml (microbiological assay) (NS). The respective mean urinary concentrations in the first 6-hour sample were 267 +/- 74 micrograms/ml and 241 +/- 58 micrograms/ml (NS). In bile, however, the microbiological assay gave higher values than HPLC:average peak concentrations of 10.3 +/- 3.4 micrograms/ml and 7.5 +/- 2.8 micrograms/ml (p less than 0.02) respectively, reached during the 2nd hour after drug administration, and mean total 24-hour biliary ciprofloxacin output of 2167 +/- 288 micrograms and 1587 +/- 222 micrograms (p less than 0.01) respectively. This may point to hepatic transformation of ciprofloxacin to more active metabolite(s) than the parent compound. The significantly higher concentrations of ciprofloxacin in bile than in serum exceeded the minimum inhibitory concentration for organisms usually responsible for biliary infections. These infections may, therefore, be favorably affected by ciprofloxacin.