To study whether individual HLA class I alleles are used preferentially or equally in human virus-specific CTL responses, the contribution of individual HLA-A and-B alleles to the human influenza virus-specific CTL response was investigated. To this end, PBMC were obtained from three groups of HLA-A and-B identical blood donors and stimulated with influenza virus. In the virus-specific CD8+ T cell population, the proportion of IFN-γ-and TNF-α-producing cells, restricted by individual HLA-A and-B alleles, was determined using virus-infected C1R cells expressing a single HLA-A or-B allele for restimulation of these cells. In HLA-B* 2705-and HLA-B* 3501-positive individuals, these alleles were preferentially used in the influenza A virus-specific CTL response, while the contribution of HLA-B* 0801 and HLA-A* 0101 was minor in these donors. The magnitude of the HLA-B* 0801-restricted response was even lower in the presence of HLA-B* 2705. C1R cells expressing HLA-B* 2705, HLA-A* 0101, or HLA-A* 0201 were preferentially lysed by virus-specific CD8+ T cells. In contrast, the CTL response to influenza B virus was mainly directed toward HLA-B* 0801-restricted epitopes. Thus, the preferential use of HLA alleles depended on the virus studied.