To identify the B- and T-cell epitopes of P1 of Haemophilus influenzae type b, 13 peptides covering 90% of the protein were chemically synthesized. Mouse, guinea pig, and rabbit antisera raised against purified native P1 were tested for their reactivities against the peptides in peptide-specific enzyme-linked immunosorbent assays (ELISAs). Six immunodominant linear B-cell epitopes were mapped to residues 103 to 137, 189 to 218, 248 to 283, 307 to 331, 384 to 412, and 400 to 437 of the mature P1 protein. When P1 peptides were screened for their reactivities with three human convalescent-phase serum specimens, peptides corresponding to residues 39 to 64, 226 to 253, and 400 to 437 reacted strongly with the antisera. Four regions (residues 39 to 64, 226 to 253, 339 to 370, and 400 to 437) contained murine T-cell epitopes. Rabbit antipeptide antisera were tested for their reactivities with the immunizing peptides and P1 protein by ELISA and immunoblots. All anti-P1 peptide antisera except those raised against peptide HIBP1-8 (residues 279 to 312) or HIBP1-8-keyhole limpet hemocyanin conjugate were shown to be specific for their respective immunizing peptides by ELISA. In addition, rabbit antisera raised against the synthetic peptides corresponding to residues 1 to 29, 39 to 64, 103 to 137, 189 to 218, 226 to 253, 248 to 283, 307 to 331, and 400 to 437 of the mature P1 protein recognized the P1 protein from both typeable and nontypeable isolates. These results suggest that these peptides contain epitopes highly conserved among typeable and nontypeable strains of H. influenzae. However, none of the antipeptide antisera have bactericidal activity, nor were they protective against H. influenzae type b in the infant rat model of bacteremia.