The glycine, taurine and sulphate conjugates of 3 beta,12 alpha-dihydroxy-5-cholen-24-oic acid were synthesized as authentic samples for the analysis of this unusual bile acid. A highly sensitive and specific quantitative assay of the bile acid and related compounds has been developed by selected-ion monitoring in gas chromatography-mass spectrometry of their methyl ester-trimethylsilyl ether derivatives, using the deuterium labelled internal standards: [2H6]3 beta,12 alpha-dihydroxy-5-cholen-24-oic acid, [2H5]3 beta-hydroxy-5-cholen-24-oic acid, [2H7]cholic acid and their sulphates. Calibration curves for these bile acids were linear over the range 0.01-100 micrograms/ml in human serum. Recoveries of the bile acids and their conjugates ranged from 95 to 103% of the added amounts of their standard samples. The unsaturated bile acid was identified in a significant amount of 25.2 micrograms/ml in serum of an infant with liver disease, and its sulphate comprised 55.1% of the amount of the bile acid.