A high-performance liquid chromatography (HPLC) assay was developed for the determination of 6 new quinolones in the plasma. The plasma samples were directly introduced onto a HPLC column after filtering through a Molcut II membrane filter, which removes high molecular weight proteins. New quinolone in filtrate was separated from interfering substances and retained on a pre-column using an ODS stationary phase and then was introduced onto an analytical column with an ODS stationary phase by column switching. New quinolones were dtected by ultraviolet absorbance in the range of 269-300 nm. Determinations of new quinolones were possible over the concentration range of 50-4000 ng/ml; the limits of detection were 20 ng/ml. The recoveries of the new quinolones added to the plasma were 96.1-101.4% with a coefficient of variation of less than 5.0%. These methods were applied to drug level monitoring in the plasma of patients treated with new quinolones and in that of healthy volunteers participating in pharmacokinetic studies. In addition, these methods were applied to a drug interaction between new quinolones and metal cation (e.g.; Mg²⁺, Al³⁺ or Fe²⁺) containing agents. Furthermore, this method was applied to the determination of skin tissue level of ofloxacin in patients after treatment with ofloxacin. A correlation between serum levels and skin tissue levels of ofloxacin was determined for 30 patients after oral administration of ofloxacin. A good correlation was obtained and the coefficient of the correlation was 0.84.