A high performance liquid chromatography (HPLC) mass spectrometry (MS) system for the analysis of glycine and taurine conjugated bile acids in human fasting and postprandial sera is described. Following purification on thin layer chromatograms conjugated bile acids were separated in a RP-18 5 microns column with methanol 0.02 M KH2PO4 buffer pH 5.10 as the mobile phase at a flow rate of 1 ml/min and assayed by direct injection MS. The bile acid fractions were assayed by MS. The amounts of conjugated bile acids in the postprandial sera of 15 subjects with normal liver function were significantly higher (p less than 0.05) than in fasting ones and the glycine/taurine ratio was about 1 for both. The method is better suited for a specific research area in experiments requiring a high level of accuracy.