Telomerase is a ribonucleoprotein complex mainly composed of a reverse transcriptase catalytic subunit (telomerase reverse transcriptase gene, hTERT) that copies a template region of its RNA subunit to the end of the telomere. For detecting telomerase activity in a tissue specimen the TRAP assay is a relatively sensitive and specific method, but it can be used only on fresh tissue extracts and offers no information at the single cell level. Immunohistochemistry (IHC) allows to detect hTERT protein expression at an individual cell level in human tissues. We have tested commercially available anti-hTERT antibodies in formalin-fixed and paraffin-embedded human tissues by IHC. Only one monoclonal antibody (NCL-hTERT; Novacastra) was sufficiently specific and this was applied to human tissues in which telomerase activity had been shown by TRAP assay and hTERT mRNA expression by RT-PCR. hTERT protein localized diffusely in the nucleoplasm and more intensely in the nucleoli of cancer cells and proliferating normal cells. Mitotic cells showed diffuse staining of the entire cell. Granular cytoplasmic staining was occasionally found in some tumor cells. In telomerase-positive tumors not all the tumor cells showed hTERT immunoreactivity. A significantly heterogeneous hTERT protein expression was observed in human tumor tissues. The hTERT immunostaining in fixed tissues was concordant with telomerase activity and hTERT mRNA expression in corresponding non-fixed samples. Quantitative RT-PCR of microdissected sections showed that hTERT mRNA expression was higher in cells with nuclear expression than in those with cytoplasmic expression. Double staining with the M30 antibody showed that a subpopulation of hTERT-negative cells is apoptotic. We conclude that: (1) hTERT protein can be detected by IHC in fixed human tissues, but the choice of the antibody, tissue processing, and reaction conditions are critical, (2) hTERT protein localizes in the nucleoplasm, more strongly in the nucleolus, and occasionally in the cytoplasm, (3) telomerase-positive tumors show significant heterogeneity of hTERT protein expression, and (4) a subpopulation of hTERT protein negative tumor cells is identified as apoptotic cells