CD44 is a glycosylated cell surface adhesion molecule expressed on a diverse range of cells and has several variant forms, some of which are involved in metastasis of cancer cells. Because little is known about CD44 in human hepatocellular carcinoma (HCC), we investigated its expression in tissue specimens from primary lesions (12 cases), in smear specimens from peritoneal effusions (2 cases), and in cell lines (HCC cell lines, KIM-1, KYN-1, KYN-2, KYN-3, HAK-1A, and HAK-1B; combined hepatocholangiocarcinoma cell lines, KMCH-1 and KMCH-2; and bile duct carcinoma cell lines, KMC-1 and KMBC). Immunohistochemical studies using monoclonal antibody recognizing epitope Group 1 of human CD44 molecule showed that HCC cells in all tissue specimens, including the original tumors of one smear specimen and HAK-1A, were negative for CD44; whereas, HCC cells in two-smear specimens, KIM-1, KYN-2, KYN-3, HAK-1A, HAK-1B, KMCH-1, KMC-1, and KMBC, showed positive reactions on the cell membrane. Immunostain-positive cell lines showed a positive cell rate of 51.9% to 99.8% by flow cytometric analysis. Western blotting detected CD44 protein of hemopoietic type in KIM-1, KYN-3, HAK-1A, and HAK-B and epithelial type in KMC-1 and KMBC. Southern blotting of complementary DNA amplified after reverse transcriptase-polymerase chain reaction (RT-PCR) detected hemopoietic type and some variant forms with longer insertion in all cell lines but KMCH-2, whereas hemopoietic type and variants with minor insertion were only detectable in tissue specimens. These findings suggest that HCC cells in ascites and in culture often express CD44, but those in tissue do not at protein level.