The pharmacokinetics of ciprofloxacin after oral and intravenous administration have been studied extensively, and the results have been published worldwide. This paper serves as a review of the pharmacokinetics of ciprofloxacin, with specific reference to its penetration into body fluids and tissues. Ciprofloxacin has a protein binding of approximately 30% and penetrates well into tissues. For instance, the total concentration (area under the serum concentration-time curve) in peripheral human lymph is 70% of the serum values, and the peak level in lymph appears with 1-2 hr. The corresponding value for suction skin blisters is 60% and for inflammatory blisters 120%. Ciprofloxacin is concentrated in white blood cells, lung, prostate, and kidney and reaches concentrations above serum in many other tissues as well. Urine concentrations are up to 100 times those in serum. The levels in bile are comparable or only slightly higher (less than or equal to x 10) than serum levels. The penetration into cerebrospinal fluid (CSF) is low; the CSF levels are 4-10% of the serum levels in noninflammatory CSF and 30-50% in CSF from patients with meningitis.