A soluble form of human leukocyte antigen class I antigen (sHLA-I) has been reported to cause apoptosis on cytotoxic T cells and inhibit killer activity of natural killer cells via killer-cell inhibitory receptors. However, its effect on cancer cells has not yet been elucidated. We examined the direct effect of sHLA-I on human liver cancer cell lines, HepG2, HLE and HLF. The effects of sHLA-I on cell growth, DNA synthesis, and apoptosis induction were evaluated. To elucidate the mechanisms, cDNA expression arrays were also examined. sHLA-I caused cell growth inhibition, resulting in apoptosis on human hepatocellular carcinoma, dose-dependently. In this process, caspase-3 was activated. sHLA-I also inhibited in vivo growth of hepatocellular carcinoma in severe combined immunodeficient mice. sHLA-I caused apoptosis on human hepatocellular carcinoma.