To recover peptides that antigenically and immunogenically mimic the p185HER2 oncoprotein, we selected the phage-peptide libraries pVIII-9aa and pVIII-9aa. Cys using murine monoclonal antibodies (mAb) MGr2 and MGr6, directed against two distinct epitopes of the p185HER2 extracellular domain. Phage-displayed peptides containing consensus amino acid motifs were recovered and shown to compete specifically for mAb binding on tumor cells that overexpress p185HER2. The deduced amino acid sequence of the peptides suggests that both epitopes defined by the mAb on p185HER2 are discontinuous and that hydrophobic interactions are involved in binding with the mAb. A phage clone displaying the GPLDSLFAQ peptide elicited a specific immune response against the p185HER2 in BALB/c mice, demonstrating that this phage-displayed peptide represents an immunological equivalent of the MGr2 epitope on p185HER2 and might be used as a substitute for this oncoprotein in in vitro and in vivo immunological studies.