A simple high-performance liquid chromatography (HPLC) assay for the simultaneous determination of guanase and aspartate aminotransferase (AST) activities in a single serum sample is described. The method is based on direct detection of enzymatically formed products xanthine and glutamate, respectively. The procedure is sensitive, precise (C.V. below 2% for guanase and 3% for AST), suitable for routine purposes and requires only 100 microliters of sample. Kinetic measurements have shown the guanase activity to have an apparent Michaelis constant of 24.5 microM and the AST activity of 11.1 and 0.18 mM for aspartate and oxoglutarate, respectively, at 37 degrees C in Tris-HCl buffer (pH 7.5).