AIM:To evaluate a culture system for bile acid formation in primary human hepatocytes in comparison with HepG2 cells.METHODS:Hepatocytes were isolated from normal human liver tissue and we re cultured in serum-free William's E medium.The medium was collected and re newed every 24 h. Bile acids and their precursors in media were finally analysed by gas chromatography mass spectrometry.RESULTS:Cholic acid (CA) and chenodeoxycholic acid (CDCA) conjugated with glycine or taurine accounted for 70% and 25% of total steroids. A third of CDC A was also conjugated with sulphuric acid. Dexamethasone and thyroid hormone alone or in combination did not significantly effect bile acid formation. The addition of cyclosporin A (10&mgr;mol/L) inhibited the synthesis of CA and CDCA by about 13% and 30%, respectively.CONCLUSION:Isolated human hepatocytes in primary culture behave as in the intact liver by converting cholesterol to conjugated CA and CDCA.This is in contrast to cultured HepG2 cells, which release large amounts of bile acid precursors and unconjugated bile acids into the medium.