Haemophilus influenzae type b is a major cause of bacterial meningitis in young children. Antibodies against the outer membrane protein P2 are protective in the infant rat model of bacteremia. To identify conserved, surface-exposed, and protective epitopes of P2, 17 overlapping peptides covering the entire sequence of the protein were synthesized. Antisera from mice, guinea pigs, and rabbits raised against chromatographically purified P2 were tested for their reactivities to the peptides by enzyme-linked immunosorbent assays (ELISA). Three major linear immunodominant B-cell epitopes were mapped to residues 53 to 81, 241 to 265, and 314 to 341 of mature P2. Human convalescent-phase antisera also reacted strongly with these three epitopes. Rabbit antisera against all peptide-keyhole limpet hemocyanin conjugates except two peptides containing residues 8 to 19 and 302 to 319 recognized the corresponding peptides in ELISA and reacted with P2 on immunoblots. Immunization with all unconjugated peptides, except the 19 N-terminal residues, induced very strong peptide-specific antibody responses, and these antisera reacted with P2 on immunoblots. Rabbit antisera raised against peptides corresponding to residues 1 to 14, 125 to 150, 193 to 219, and 241 to 319 also recognized P2 purified from H. influenzae nontypeable isolates. Identification of these immunodominant B-cell epitopes and conserved regions is a first step toward the rational design of a universal H. influenzae vaccine.