A simple and highly sensitive capillary electrophoresis (CE) method for determining the content of N-acetylneuraminic acid (Neu5Ac) in glycoproteins was developed. Neu5Ac was derivatized with 2-aminoacridone (AMAC) by reductive amination, and the AMAC-Neu5Ac adduct could be readily separated from the other 11 AMAC-derivatized neutral and acidic monosaccharides usually present in glycoproteins by CE in a 0.3 mol/L borate buffer, pH 10.5, and detected at 260 nm. The derivatization of Neu5Ac was achieved at 55 degrees C for 4 h. AMAC-Neu5Ac was stable at 20 degrees C in the dark for at least 12 h while at room temperature it spontaneously converted into another substance with a lower electrophoretic mobility, which was identified as decarboxylated AMAC-Neu5Ac by high performance liquid chromatography - ion trap mass spectrometry (HPLC-ITMS). Concentration and mass of Neu5Ac as low as 1 micromol/L and 35 fmol could be detected. The linear correlation coefficient between the ratio of peak area to migration time of AMAC-Neu5Ac and the concentration of Neu5Ac ranging from 10 to 120 micromol/L was 0.9978 (n=8). This method was successfully applied to the analysis of sialic acid in human urinary trypsin inhibitor (hu-UTI), bovine alpha1-acid glycoprotein (alpha1-AGP) and recombinant human erythropoietin (rhu-EPO). By combination of CE and HPLC-ITMS we found that N-glycolylneuraminic acid (Neu5Gc) was present in bovine alpha1-AGP in addition to Neu5Ac, with a quantity comparable to that of the latter.