Albumin is a ubiquitous protein synthesized only by hepatocytes. Detection of albumin messenger RNA (mRNA) in neoplastic and non-neoplastic tissues using in situ hybridization may be useful in demonstrating hepatocellular differentiation. We investigated the presence of albumin mRNA by in situ hybridization in 69 hepatic and 29 extrahepatic tumors to determine its sensitivity and specificity for hepatocellular differentiation. Albumin mRNA was detected in 22 of 23 hepatocellular carcinomas (96%); all hepatoblastomas, hepatocellular adenomas, and focal nodular hyperplasias; and within the hepatoid areas of an extrahepatic malignant mixed germ cell tumor. All other hepatic and extrahepatic tumors were negative for albumin mRNA. The staining was intense in focal nodular hyperplasias, weak in hepatocellular adenomas, and variable in hepatocellular carcinomas and hepatoblastomas. Non-neoplastic, noncirrhotic liver was present in 50 cases; all were positive for albumin mRNA with variable staining, which was usually more intense than in the neoplasms. A zonal pattern of staining was present in 49 (98%) of these, with more intense staining within acinar zone 1 and least in zone 3. This study shows that albumin mRNA is a specific marker of hepatocellular differentiation but may be present in extrahepatic germ cell tumors with hepatoid features. For primary hepatocellular carcinomas, in situ hybridization for albumin mRNA is a useful adjunct diagnostic method.