A panel of 2 polyclonal and 7 monoclonal antibodies directed against cytokeratins was tested on cryostat and paraffin sections of 14 normal human liver biopsies using an immunoperoxidase procedure. The staining characteristics of hepatocytes and bile ducts are reported. On cryostat sections, monoclonal antibodies directed against individual cytokeratins no. 8 and no. 18 stained both bile ducts and hepatocytes, whereas monoclonals anti-cytokeratin no. 7 and no. 19 exclusively stained bile ducts. The potential use of these 4 monoclonal antibodies in liver histopathology is briefly discussed. Monoclonal antibody anti-type II cytokeratins and the polyclonal rabbit anti-human keratin stained only bile ducts on both cryostat and paraffin sections. Using monoclonal antibody CAM 5.2 on paraffin sections, both bile ducts and parenchyma were positive. An acinar gradient was apparent in that zone 1 hepatocytes were more intensely stained. Moreover, a rim of hepatocytes around terminal hepatic venules and adjacent to subhepatic veins showed more intense staining. The same gradient could be seen in some paraffin sections stained with the monoclonals anti-cytokeratin no. 18 and KL1, and the rabbit polyclonal anti-keratin "wide spectrum screening". The gradient is interpreted as reflecting quantitative differences in keratin content between hepatocytes. Polyclonal rabbit anti-human keratin is proposed as the most reliable antibody for identification of bile ducts in paraffin sections. The usefulness of reliable bile duct staining in several pathological conditions is emphasized.