It is commonly perceived that the human immune system is naive to the newly emerged H5N1 virus. In contrast, most adults have been exposed to influenza A H1N1 and H3N2 viruses through vaccination or infection. Adults born before 1968 have likely been exposed to H2N2 viruses. We hypothesized that CD4(+) T cells generated in response to H1N1, H3N2, and H2N2 influenza A viruses also recognize H5N1 epitopes. Tetramer-guided epitope mapping and Ag-specific class II tetramers were used to identify H5N1-specific T cell epitopes and detect H5N1-specific T cell responses. Fifteen of 15 healthy subjects tested had robust CD4(+) T cell responses against matrix protein, nucleoprotein, and neuraminidase of the influenza A/Viet Nam/1203/2004 (H5N1) virus. These results are not surprising, because the matrix protein and nucleoprotein of influenza A viruses are conserved while the neuraminidase of the H5N1 virus is of the same subtype as that of the circulating H1N1 influenza strain. However, H5N1 hemagglutinin-reactive CD4(+) T cells were also detected in 14 of 14 subjects examined despite the fact that hemagglutinin is less conserved. Most were cross-reactive to H1, H2, or H3 hemagglutinin epitopes. H5N1-reactive T cells were also detected ex vivo, exhibited a memory phenotype, and were capable of secreting IFN-gamma, TNF-alpha, IL-5, and IL-13. These data demonstrate the presence of H5N1 cross-reactive T cells in healthy Caucasian subjects, implying that exposure to influenza A H1N1, H3N2, or H2N2 viruses through either vaccination or infection may provide partial immunity to the H5N1 virus.