Hepatic bile acid concentrations are elevated in chronic cholestasis because of reduced canalicular excretion and active ileal absorption of the fraction eliminated in the gut. Ursodeoxycholic acid (UDCA) reduces the intestinal absorption of endogenous bile acids, thereby diminishing the concentrations to which liver cells are exposed. In the isolated perfused liver (in which vectorial bile acid transport is maintained), UDCA reduces the cytotoxic and cholestatic effects of endogenous bile acids. As a result, it has been suggested that UDCA or one of its conjugates could have a direct protective effect on hepatocyte structure and function. We therefore studied the effects of chenodeoxycholic acid (CDCA) and tauroursodeoxycholic acid (TUDCA) alone and in combination on the viability and certain functions of human hepatocytes in primary culture. TUDCA did not affect intracellular concentrations of CDCA when added concomitantly. In other experiments, CDCA (100 to 500 mumol/L) induced concentration-dependent increases in lactate dehydrogenase (LDH) leakage and decreases in cellular protein synthesis and albumin secretion. Neither TUDCA nor UDCA had similar effects at the same concentrations, nor did they have a protective effect when added concomitantly with CDCA at equimolar or twice-equimolar concentrations. These results suggest that UDCA has no direct cytoprotective effect when the bile acid concentrations to which human hepatocytes are exposed are unchanged. They also suggest that the hepatoprotective effect of UDCA in cholestatic human liver diseases and in the isolated perfused liver loaded with hydrophobic bile acids occurs through its effect on intestinal and hepatocyte transport systems.