Immunohistochemical techniques were used to study 177 hepatic tumors (hepatocarcinoma, cholangiocarcinoma, hepatocholangiocarcinoma, adenocarcinoma of unknown origin, and metastatic carcinoma). Phenotypes suggestive of hepatocarcinoma included keratins 8 and 18, factor XIII a, alpha-fetoprotein. C-reactive protein, carcinoembryonic antigen (CEA) cross-reacting antigen; those in effect that excluded hepatocarcinoma were keratins 1, 5, 10, 11, 19, true CEA. C-reactive protein, used for the first time, proved to be a fairly sensitive and specific marker. Factor XIII a, which was thought to be synthesized only by histiocytes, was also present in hepatocytes. Immunohistochemistry appears to be an important tool in the diagnosis of hepatic tumors. As a result of this study, 32 cases were reclassified; several were found to be intermediate between hepatocarcinoma and cholangiocarcinoma. Sixteen cases apparently were true hepatocholangiocarcinomas. In 12 cases of hepatocarcinoma, some tumor cells expressed keratins of bile duct type. It was impossible to differentiate immunohistochemically cholangiocarcinoma from metastatic carcinoma, except in two cases with breast tissue markers.