A rapid, specific procedure is described for the quantitation of ketoconazole in biological fluids using high-performance liquid chromatography (HPLC). The procedure involves sample preparation using a reverse-phase C-18 cartridge prior to chromatography and quantitation using peak height ratios (UV absorbance detection, 231 nm) of ketoconazole to the internal standard, phenothiazine. A sensitivity of 0.2 micrograms/ml was achieved using a 0.5-ml sample. The mean recovery was 86.2%, and overall coefficient of variation of the procedure was 7.1%. This procedure has been used to determine ketoconazole levels in human serum, plasma, CSF, and synovial fluid. A comparison with a microbiological assay is presented, and adaptability of this procedure to quantitation by fluorescence to increase the sensitivity fivefold is discussed.