Evidence suggests that cellular immunity to hepatitis C virus (HCV) core protein may be important in the pathogenesis of viral infection. Therefore, interferon gamma (IFN-γ) production by peripheral blood mononuclear cells (PBMC) derived from patients with chronic HCV infection (genotype 1b) was examined. The cellular immune response was evaluated with a recombinant HCV core fusion protein derived from a patient with genotype 1b. To identify the immunodominant epitopes, IFN-γ production in responders was also assessed with a panel of nine synthetic peptides that covered the entire core region. It was found that mononuclear cells from 24 (52%) of 46 patients with chronic liver disease responded to the core protein; asymptomatic HCV carriers demonstrated a lower response rate (14%, P < .05). More important, individuals who had received IFN-α treatment and went into clinical and virological remission had a higher response rate (75%, P < .05) compared with those with ongoing hepatitis whose treatment failed (31%). Of 25 patients whose mononuclear cells responded to HCV core protein, 18 had a significant response to one or more peptides; 12 patients reacted to a peptide mixture containing hydrophilic sequences. The core peptide amino acid sequence 141 to 160 was recognized in 9 patients. Interestingly, 7 of 8 patients bearing HLA DR 4 and w53 haplotypes recognized the peptide sequence 141 to 160. Thus, IFN-γ production of the mononuclear cell response appeared to be HLA DR restricted, and the responding cells were identified as CD4+ T cells. This study suggests the presence of immunodominant T cell epitopes within the HCV core protein in association with HLA DR phenotypes in patients with HCV-associated liver disease. (HEPATOLOGY 1995;22:1057–1064.).