A simple, selective and sensitive method has been developed to determine ciprofloxacin in human aqueous humor. Separation of ciprofloxacin was carried out with pipemidic acid as internal standard using a Novapak C-18 reversed-phase cartridge column (100 x 8 mm i.d., particle size 4 mu m) and a mobile phase consisting of methanol-acetonitrile-citric acid (0.4 M) (3:1:10, v/v/v) at a flow rate of 1 ml min(-1). The column effluent was monitored with fluorescence detection at 278 nm (excitation) and 450 nm (emission) after direct injection. The retention times were 4.88 min for pipemidic acid and 7.52 min for ciprofloxacin. The within-day and day-to-day reproducibilities were less than 7% for ciprofloxacin at 0.1 and 1 mu g ml(-1) (n = 6). The mean recovery from aqueous humor was found to be 101.37 +/- 6.7% for ciprofloxacin at 0.1 mu g ml(-1) (n = 6) and the detection limit corresponding to a signal-to-noise ratio of 2.5:1 was 250 pg ml(-1). The method was shown to be suitable for determining ciprofloxacin levels in human aqueous humor samples.