Presence of enrofloxacin residues in fish liver, kidney and muscle tissue was investigated after per os application of the drug. For the purpose of determination of enrofloxacin, the following analytical methods were used: microbiological method - plate pH 8 with Escherichia coli ATCC 11303 and HPLC method with fluorescence detection. After a 5-day oral treatment of carps, enrofloxacin residues in tissues were determined up to the 10th day after the end of the drug application. Enrofloxacin content determined by the HPLC method was lower than MRL; drug residues were determined in liver on the 6th day after treatment, in kidney on the 7th day and in muscle on the 9th day after treatment. The results of enrofloxacin residues determination by screening method on the medium with E. Coli ATCC 11303, pH 8 show that this procedure can be used for qualitative determination of enrofloxacin. The screening method allows determination of enrofloxacin in fish tissues below the MRL. Cyprofloxacin was not detected in fish liver, kidney and muscle tissue.