Integrin a 5β1 and a 2β1 are the major integrin receptors in human hepatocytes. However, in human hepatocellular carcinoma cells it was found that the expression of integrin a 5β1 was decreased and another integrin a 6 β1 increased. In this study, the SMMC7721 human hepatocellular carcinoma cells cotransfected or singlely transfected with integrin a 5 and/or β 1 cDNAs were established, and designated a 5β 1.6-7721, a 5.3-7721, and β 1.6-7721 cell lines, respectively. Transfection with cDNAs of integrin a 5 and β 1 subunits resulted in the over expression of each integrin and modified biological properties, including a slowed growth rate, changes in the cell cycle from 15.5% of control cells in the G2/M phase to 12.1%, 9.6% and 9.4% in a 5.3-7721, β 1.6-7721, a 5 β 1.6-7721, respectively, and a decrease in the Cell Mitosis Index from 1.6 in controls to 0.96, 0.95, and 0.72, and 34%, 28% and 52% derived from colony forming ability, respectively. Tumorigenicity was also tested in nude mice with inoculation of cells subcutaneously. Tumor masses growing in nude mice following inoculation with b 1.6-7721, and a 5 β 1.6-7721 cells weighed only 52% or 31% those of control cells. These results indicated that deletion or low expression of integrin a 5 β 1 may play an important role in the development of hepatocellular carcinoma. Therefore, induction of expression of the integrin a 5 β 1 in malignant cells could be a potential means of treating hepatocellular carcinoma. (Mol Cel Biochem 207: 49-55, 2000)