Quantification of outputs of antibacterial agents from fish farms is necessary to assess their environmental impact. In effluents, antibacterial agents are dissolved or associated with mineral or organic materials, the heaviest of which quickly settle at the exit of the fish farm. Oxolinic acid (OA) is one of the quinolones the most frequently used in turbot farms. For the first time, procedures are described for determination of dissolved, suspended, and settled forms of OA by reversed-phase high-performance liquid chromatography. Chromatography was performed on a 5 mu m C-18 cartridge, with a mixture of acetonitrile and aqueous orthophosphoric acid solution as mobile phase, and fluorescence detection. OA was easily and completely isolated by solid-liquid extraction. The method is selective, accurate, and precise; limits of quantitation were 0.005 mu g mL(-1), 0.025 mu g per filter, and 0.025 mu g g(-1), respectively for dissolved, suspended, and settled forms of OA. These methods enabled initial assessment of the amounts of dissolved, suspended, and settled forms of OA (72, 23, and 2%, respectively) of the antibacterial agent administered.