Male A/JCr mice with naturally occurring Helicobacter hepaticus infection develop a progressive chronic active hepatitis and liver tumors, despite the presence of serum antibodies to Helicobacter proteins. A rabbit antiserum prepared against the bacterial proteins immunoreacted with hepatocytes present in liver sections from infected mice with progressive lesions. We found that sera from these mice contained IgG antibodies that reacted in immunoblots with recombinant heat shock protein 70 (DmaK from Escherichia coli) but not with heat shock protein 60 (GroEL) or heat shock protein 10 (GroES). A rabbit antibody to heat shock protein 70 reacted with H. hepaticus in tissue sections and to a H. hepaticus protein (70 kd) in Western blots. Immunohistochemistry and in situ hybridization for heat shock protein 70 revealed that individual hepatocytes and other cells expressed the protein in livers with hepatitis but not usually in normal livers. Liver tumors and preneoplastic lesions in infected mice did not usually express heat shock protein 70 except focally in a few tumors. In situ hybridization for H. hepaticus 16S rRNA showed that the bacteria was found throughout the liver associated with hepatitis but not within tumors. CD3+ T lymphocytes were found in close association with hepatic lesions. These data suggest a role for autoimmunity in progressive hepatitis and carcinogenesis in livers infected with H. hepaticus.