OBJECTIVE In cholestatic liver disease, bile acids may initiate or aggravate hepatocellular damage. Cellular necrosis and cell death may be due to detergent effects of bile acids, but apoptosis may also play a role. In cholestasis, the conditions determining either apoptotic or cytolytic cell death are still unclear. Primary rat hepatocytes in culture represent a suitable model to study bile-acid-induced liver damage. METHODS Glycochenodeoxycholic acid, a hydrophobic bile acid, was used to induce cell damage. Tauroursodeoxycholic acid, a hydrophilic bile acid, served as substrate to study possible protective effects of such compounds. To study the time and concentration dependency of bile-acid-induced cytolysis and apoptosis, morphologic alterations, hepatocellular enzyme release and nucleosomal DNA fragmentation were evaluated. RESULTS Bile-acid-induced cytolysis, as indicated by hepatocellular enzyme release and by morphologic signs of membrane destruction, increased with concentration and time. Addition of tauroursodeoxycholic acid to the incubation medium reduced cytolysis significantly, indicating a direct hepatoprotective effect of this bile acid against the detergent action of hydrophobic bile acids. In contrast to cytolysis, apoptosis with DNA fragmentation was induced by low concentrations of glycochenodeoxycholic acid a few hours after incubation. Coincubation with tauroursodeoxycholic acid in equimolar concentrations significantly reduced apoptosis, indicating another direct hepatoprotective effect of tauroursodeoxycholic acid. CONCLUSIONS It seems likely that in severe cholestasis, bile-acid-induced injury of hepatocytes is due mainly to cytolysis, whereas in moderately severe cholestasis apoptosis represents the predominant mechanism of bile acid toxicity. Tauroursodeoxycholic acid may reduce both bile-acid-induced apoptosis and cytolysis.