An assay for glycine and taurine conjugates of cholic (GC, TC), chenodeoxycholic (GCDC, TCDC), deoxycholic (GDC, TDC) and ursodeoxycholic (GUDC, TUDC) acids in human serum by high-performance liquid chromatography using an immobilized 3-alpha-hydroxysteroid dehydrogenase post-column reaction and an off-line spectrofluorimetric detection is described. The detection limit is 0.2 microM. Recoveries of extraction and chromatographic analysis range from 89.50 to 91.68% and from 96.75 to 98.83%, respectively. The analysis in serum reveals a coefficient of variation for reproducibility between 16.2% for GC and 4.7% for TDC; inter-assay variability ranges between 4.06% for TCDC and 8.24% for TUDC.