Telomerase, a ribonucleoprotein enzyme associated with cellular immortality, consists of human telomerase RNA component (hTERC), human telomerase protein 1 (hTEP1), and human telomerase reverse transcriptase (hTERT). In this study, the expression of these subunits was examined in non-neoplastic livers [13 cases of chronic viral hepatitis (CVH), 16 of primary biliary cirrhosis (PBC), two of primary sclerosing cholangitis, and six normal livers], using the reverse transcription-polymerase chain reaction (RT-PCR), nested PCR, and in situ hybridization (ISH). Six hepatocellular carcinoma (HCC) cases and one colonic cancer were used as positive controls. Telomeric repeat amplification protocol (TRAP) assay disclosed distinct telomerase activity in all positive controls and weak telomerase activity in non-neoplastic livers in 4 of 13 CVH cases and 5 of 16 PBC cases. By RT- and nested PCR, both hTERC and hTEP1 mRNA were detectable in all non-neoplastic liver tissues; ISH revealed hTERC and hTEP1 mRNA in the periportal and periseptal hepatocytes and inflammatory mononuclear cells in those cases examined. ISH revealed hTERT mRNA only in a few infiltrating mononuclear cells in 3 of 13 CVH and 2 of 16 PBC livers and these five cases were also positive by TRAP assay. In four of these five cases, hTERT mRNA was also detectable by nested PCR, suggesting that hTERT mRNA in the non-neoplastic liver is expressed by infiltrating mononuclear cells. Biliary epithelial cells were totally negative for these human telomerase subunits. Three subunits were constantly detected in all positive controls by ISH as well as by RT- and nested PCR. The finding that hTERC and hTEP1 mRNA, but not hTERT mRNA, were detectable in the non-neoplastic hepatocytes suggests that telomerase is present but not activated and that additional factor(s) are necessary for the expression of hTERT mRNA in the hepatocytes, along with immortalization and neoplastic transformation.