Hepatocyte monoclonal antibody (Hep) (alternatively Hep Par 1 for Hep paraffin 1) has been reported to stain normal hepatic tissue and hepatocellular carcinoma (HCC) with high specificity. We have studied the Hepatocyte expression in 96 cases of HCC and 311 cases of nonhepatic epithelial tumors. All cases of HCC were also stained with CEA-Gold 5, CD10, and alpha-fetoprotein. Hep, CEA-Gold 5, CD10, and alpha-fetoprotein immunostains were performed on formalin-fixed, paraffin-embedded tissue sections. Hep immunoreactivity was detected in 88 of 96 cases of HCC (92%), with a cytoplasmic and granular pattern of staining. The level of Hep expression in HCC corresponded to the nuclear grade and growth pattern. All 50 cases of nuclear grade 1 and nuclear grade 2 HCC were positive (100%), whereas 37 of 44 nuclear grade 3 (84%) and 1 of 2 nuclear grade 4 (50%) were positive. Sixty-seven of 68 cases of HCC with a trabecular, pseudoglandular, or scirrhous growth pattern were positive (98%), whereas 22 of 27 cases of HCC with a compact growth pattern were positive (81%). CEA-Gold 5, CD10, and alpha-fetoprotein immunoreactivity was detected in 76% (73 of 96), 52% (50 of 96), and 31% (30 of 96) cases of HCC, respectively. The positive predictive value of the combination of all four antibodies was 97%. Three cases of HCC were negative for all four antibodies; these cases had a high nuclear grade or a sarcomatoid or compact growth pattern. Twenty of 311 cases of nonhepatic tumors were positive for Hep (6%): 15 were adenocarcinomas and five were neuroendocrine tumors. The negative predictive value of Hep in HCC was 94%. The Hep-positive nonhepatic epithelial tumors were easily distinguished from HCC by the expression of keratin 7 or keratin 20 for adenocarcinoma and chromogranin and synaptophysin for neuroendocrine tumors because HCC does usually not express these markers. With the exception of two cases of hepatoid gastric carcinoma, all Hep-positive nonhepatic epithelial tumors were negative for alpha-fetoprotein, CEA-Gold 5, and CD10. Our study demonstrates that Hep is a relatively specific marker for HCC. It is useful in differentiating HCC from primary hepatic cholangiocarcinoma and metastatic tumors when combined with other immunomarkers.