We characterized the human CD8⁺ T cell response against influenza A viruses by a flow cytometry–based assay. Peripheral blood mononuclear cells (PBMCs) were incubated with inactivated influenza virus preparation, for 17 h, and were stained for intracellular interferon-γ. Major histocompatibility complex class I–restricted memory CD8⁺ T cells specific for influenza antigens were detected in PBMCs from all 19 adult donors, at an average frequency of 0.39%. On average, 83% of influenza virus–specific CD8⁺ T cells expressed the differentiation-associated marker CD27, a percentage that is significantly higher than that of CD8⁺ T cells specific for pp65 of human cytomegalovirus (53%). These observations indicate that class I–restricted immunity against influenza A viruses is characterized by the persistence, after clearance of infection, of circulating antigen-specific CD8⁺ T cells. The different patterns of CD27 expression in influenza virus– and cytomegalovirus-specific CD8⁺ T cells suggest that influenza virus–specific memory and effector CD8⁺ T cells can be differentiated by phenotypic analysis