Apoptosis occurs naturally in the liver and increases in specific pathogenic processes. We previously described the use of a chemically defined medium supplemented with epidermal growth factor and dimethylsulfoxide to maintain rat hepatocytes in a highly differentiated state for more than 30 days (long-term culture). In this study, we showed that hepatocytes in long-term dimethylsulfoxide culture have definite advantages over using cells in short-term culture (cells in culture for 2 to 4 days) to study apoptosis. We demonstrated that treatment with tumor necrosis factor (TNF)-alpha induced apoptosis (detected morphologically and by formation of an oligonucleosomal DNA ladder) only in hepatocytes that had been subjected to dimethylsulfoxide removal. Neither treatment with TNF-alpha alone or dimethylsulfoxide removal alone induced apoptosis. Apoptosis could be induced by concentrations as low as 500 U of TNF-alpha/ml. Although a DNA ladder was not detected by 12 hours after TNF-alpha treatment, it was easily identified by 24 hours. We conclude that this system can be used 1) to examine the underlying mechanism by which TNF-alpha causes apoptosis in hepatocytes and 2) to study induction of apoptosis in hepatocytes by other agents.