The hemagglutinin protein (HA) of the influenza virus family is a major antigen for protective immunity. Thus, it is a relevant target for developing vaccines. Here we describe a human CD4(+) T cell epitope in the influenza HA that lies in the fusion peptide of the HA. This epitope is well conserved in all 16 subtypes of HA of influenza A and the HA of the influenza B virus. By stimulating peripheral blood mononuclear cells (PBMCs) from a healthy adult donor with peptides covering the entire HA protein based on the sequence of A/Japan/305/1957 (H2N2), we generated a T cell line specific to this epitope. This CD4(+) T cell line recognizes target cells infected with influenza A virus seasonal H1N1 and H3N2 strains, a reassortant H2N1 strain, the 2009 pandemic H1N1 strain and influenza B virus in cytotoxicity assays and intracellular cytokine staining assays. It also lysed target cells infected with avian H5N1 virus. We screened healthy adult PBMC for T cell responses specific to this epitope and found individuals who have ex vivo IFN-γ responses to this peptide epitope in ELISPOT assays. Almost all donors that responded to this epitope have the HLA-DRB1*09 allele, a relatively common HLA allele. Although natural infection or standard vaccination may not induce strong T and B cell responses to this highly conserved epitope in the fusion peptide, it may be possible to develop a vaccination strategy to induce these CD4(+) T cells which are cross-reactive to both influenza A and B viruses.