Epitopes defined by monoclonal antibodies (mAb) specific for the Bordetella pertussis outer membrane protein P.69 (pertactin) were mapped using a series of amino- and carboxy-terminal deletion mutants expressed in Escherichia coli. mAb were found to bind predominantly to a region of pertactin spanning a (Pro-Gln-Pro)5 repeat motif and one mAb was found to bind to another region spanning a (Gly-Gly-Xaa-Xaa-Pro)5 repeat motif. To localize further the mAb-binding sites, a panel of synthetic peptides, a series of 94 overlapping hexameric peptides, and a P.69 30-amino acid fusion to a hepatitis B core protein (HBcAg-69), were synthesized. This combined approach has identified the binding site for the mAb BBO5: Pro-Gly-Pro-Gln-Pro-Pro; mAb BBO7, E4A8 and E4D7: Ala-Pro-Gln-Pro-Pro-Ala-Gly-Arg; and mAb BPE3: Thr-Leu-Trp-Tyr-Ala-Glu-Ser-Asn-Ala-Leu-Ser-Lys-Arg. We have used a non-lethal murine respiratory model of B. pertussis infection to investigate the ability of a peptide containing the epitope of the mAb BBO5 to elicit protective immunity. Immunization of mice with the HBcAg-69 protein prevented growth of B. pertussis in the lungs compared to mice receiving HBcAg alone, and protection correlated with high titers of anti-P.69 antibodies.