The cytotoxicity of a commercial PCB mixture, Aroclor 1254, was assessed on cultured foetal rat hepatocytes. Under control conditions, dexamethasone stimulates immature hepatocytes to differentiate into both hepatocytes and biliary epithelial cells. Consequently, foetal rat hepatocytes maintain, in vitro, a liver-like organization with spaces corresponding to the lumen of biliary canalicules, many mitochondria, and a well-developed rough endoplasmic reticulum (RER). This in vivo-like organization of cultured rat hepatocytes remains unchanged in medium supplemented with Aroclor 1254 at concentrations below 25 microM. In the 25-125 microM concentration range, however, PCBs severely alter some cellular organelles, notably causing important development of the RER and the appearance of cytoplasmic lacunae containing laminated concentric membrane arrays. In addition, the number of lipid droplets increases, the glycogen islets disappear, and dramatic local alterations of the mitochondrial cristae occur. In exposed and unexposed cells, the following biochemical parameters were measured: the DNA content, protein synthesis, lipid peroxidation, and urea formation. The results show that Aroclor 1254 at concentrations exceeding 25 microM (but not at lower concentrations) causes irreversible damage to cultured hepatocytes. The observed ultrastructural modifications are in good agreement with several in vivo studies on rat liver. Thus, isolated foetal rat hepatocytes have considerable potential as an alternative to whole animals for use in (eco)toxicological studies.