A sensitive method for the determination of DQ-2556 by high-performance liquid chromatography was established. The limits of detection for serum and urine were 0.1 and 2 micrograms/ml, respectively. Two clean-up procedures for serum samples were developed. In the first, deproteinization with 10% trichloroacetic acid was used and the recovery was 68.5%. In the other, ultrafiltration under acidic conditions was employed and the recovery was 85.1%. The former procedure is economical but complicated, whereas the latter is simple and labour-saving, but a special ultrafiltration tube is required. This situation offers a flexible choice, depending on the conditions of the laboratory.