Analysis of epitopes recognized by therapeutic monoclonal antibodies (mAb) is critical in clinical applications and in structure/function studies of target antigen. mAb MGr6 recognizes the extracellular domain of the p185HER2 oncoprotein and is a promising candidate for cancer immunodiagnosis and immunotherapy. Thus, epitope location and structure on p185HER2 need to be investigated. The use of MGr6-selected phage-displayed peptides for epitope analysis served to dissect the MGr6 epitope into at least two subregions, mimicked by CHSDC- and (L)P-(L)K(L) phage displayed peptides, respectively. Comparison of amino acid sequences of CHSDC peptides with the p185HER2 protein sequence and analysis of MGr6 reactivity with p185HER2 deletion mutants identified the linear subregion CCHEQCAAG of the MGr6 epitope, corresponding to amino acids 235-243 of the p185HER2 protein. This continuous subregion is part of a larger conformational epitope, and other amino acids, including a proline, a lysine and leucine residues contained in (L)P-(L)K(L) phage-displayed peptides appear to contribute to the formation of the MGr6 epitope surface.