In situ hybridization was used to detect albumin mRNA in normal liver and hepatocellular neoplasms in 20 male B6C3F1 mice between 17 and 24 months of age. Positive signals for albumin were observed consistently in the cytoplasm of hepatocytes in normal liver, particularly in periportal areas. No signals were observed in other cells, such as Kupffer's cells, mesenchymal cells, or bile duct epithelium. Of hepatocellular adenomas, 11/11 (100%) stained positively for albumin mRNA, whereas 14/15 (93%) of primary hepatocellular carcinomas showed positive expression. Albumin mRNA was also detected in extrahepatic metastases of hepatocellular carcinoma, including 9/15 (60%) of pulmonary neoplasms and 5/12 (42%) of metastases at other sites. The pulmonary metastases of hepatocellular carcinoma frequently exhibited a glandular, papillary, or sarcomatous histologic appearance. The presence of albumin in these tumors, lacking characteristic hepatocellular phenotype, is a potential determinant of hepatic lineage. We conclude that in situ hybridization for albumin mRNA in mice is a useful tool in the differential diagnosis of hepatocellular carcinoma, particularly in the case of pulmonary metastasis. This technique may also enable recognition of hepatocyte differentiation in glandular structures with phenotypic features of biliary cells, as seen in mixed hepatocellular-cholangial neoplasms.