It has been reported that the cytokeratin 19 (CK19) fragment, CYFRA 21-1, is increased in sera of some hepatocellular carcinoma patients. We hypothesized that CYFRA 21-1 might be released directly from hepatocellular carcinoma. To investigate this mechanism, we evaluated expression of CK19 in 8 human cell lines by immunoradiometric assay, Western blotting, immunohistochemistry, and reverse transcriptase-polymerase chain reaction in Chang liver cells, 2 cholangiocarcinoma cell lines, and 5 hepatocellular carcinoma cell lines. Three of 5 hepatocellular carcinoma cell lines released CYFRA 21-1, synthesized CK19 protein and expressed mRNA for CK19, as observed in 2 cholangiocarcinoma cell lines. In contrast, there was no expression of CK19 in Chang liver cells or 2 of 5 hepatocellular carcinoma cell lines. Analysis of genomic DNA for CK19 demonstrated that exon 1 was not amplified in the 3 cell lines not expressing CK19. However, there were no point mutations within exon 1 and the promoter region of CK19 in hepatocellular carcinoma cell lines. Our present study demonstrates that: i) the expression of CK19 was evident in some human hepatocellular carcinoma cell lines, and ii) the expression of mRNA for CK19 was related to the release of CYFRA 21-1. These results partially clarify the mechanism by which some hepatocellular carcinoma cell lines produce CK19 and others do not.