To investigate the role that Fas ligand plays in the apoptosis of hepatocytes induced by hepatitis C virus infection, we isolated a cDNA clone for human Fas ligand and examined the expression of Fas ligand in liver-infiltrating mononuclear cells obtained from patients with chronic hepatitis C. The amino acid sequence of human Fas ligand showed 76% and 77% identity with those of rat and mouse Fas ligand, respectively. When the expression of Fas ligand transcripts was tested by reverse transcription-polymerase chain reaction, the amplified signal was detected in liver-infiltrating mononuclear cells and peripheral blood mononuclear cells, whereas only a weak signal or none at all was detected in liver tissues. These findings suggest that the Fas ligand-Fas antigen system may play an important role in liver cell injury by hepatitis C virus infection.