Peripheral blood mononuclear cells (PBMC) contain NK cells, cytotoxic T-lymphocytes (CTL), helper T-cells, and B-cells that respond to viral infection and act to eliminate the virus from infected individuals. CTLs are not only thought to be a major host defense against viral infection, but are also implicated in the immunopathogenesis. Classical CTLs are CD8+ and recognize endogenously synthesized and processed antigen in association with a human leukocyte antigen (HLA) class I molecule. The antigens are usually 8–10 amino acids long. HCV-specific CTLs have been demonstrated in the peripheral blood of some of patients with HCV infection by stimulating PBMC with the HCV synthetic peptides (1 ). The peptides were synthesized as overlapping peptides to encompass a certain region of the HCV antigen (1 ), on the basis of antigenicity prediction from the amino acid composition of HCV (2 ), or on the basis of the HLA binding motifs in the HCV antigen (3 ). Several minimal and optimal epitopes in the HCV antigen and their HLA restriction of recognition by CTLs have been defined. Recently, it has been reported that HCV-specific CTLs may suppress the outgrowth of HCV (4 ). In this chapter, methods will be discussed that demonstrate HCV-specific CTLs in the peripheral blood of patients with HCV infection. We use nonradioisotope europium (Eu) for assay of CTL activities.