To identify the immunodominant epitopes in the HA protein, a peptide scanning assay was performed. A set of 50 peptides with five residues overlapping with the adjacent peptides spanning the ectodomain sequences of the HA protein of the H1N1pdm virus strain A/California/04/2009 were synthesized. The binding between these peptides and the convalescent serum samples from 11 H1N1pdm patients were examined by ELISA using these peptides as coating antigens. Five of these peptides (P3, P5, P15, P16 and P31) were found to react well with the sera tested. These peptides corresponded to the sequences of amino acid (aa) residues 38-52, 58-72, 158-172, 168-182, and 318-332 in the HA molecule, respectively ( Fig. 1A and Table 1 ). Among them, the P3 peptide reacted with 54.5% (6/11) of the sera, the P15 and P16 peptides reacted with 81.8% (9/11) of the sera, while the P5 and P31 peptides reacted with 100% (11/11) of the sera. These data indicated that these peptides may contain H1N1pdm virus B cell epitopes.