Nontypeable Haemophilus influenzae (NTHI) is a common cause of otitis media in children and lower respiratory tract infection in adults with chronic lung disease. The highly conserved P6 protein of NTHI infection is under evaluation as a vaccine antigen in several animal models. To elucidate the role of cellular immune response to P6 in protective immunity, the goal of this study was to identify and characterize T cell epitope(s) on P6 and to investigate the role of these epitope(s) in eliciting antigen specific antibody responses and in mediating pulmonary clearance of NTHI. We report that T cells from BALB/c immunized with P6 recognize a single, immunodominant region, represented by 15 amino acids (residues 41-55) of the P6 protein. To verify the ability of this epitope to elicit T cell responses to the P6 protein, mice were immunized with a synthetic peptide corresponding to the sequence of dominant peptide. T cells isolated from mice primed in vivo with the peptide responded following in vitro stimulation with either the peptide or with the whole P6 molecule. Substitution of single amino acids and N or C terminal truncations of the dominant peptide resulted in complete abrogation of the response, implicating their importance to the T cell response. Furthermore, mucosal immunization of mice with a chimeric peptide that encompassed the dominant T cell epitope and a putative B cell epitope resulted in enhanced bacterial clearance following pulmonary challenge with NTHI. Collectively, these results establish that, in a mouse model, P6 contains a single immunodominant T cell epitope and this epitope plays an important role in protective immune responses induced by immunization with P6.