To screen the H1-subtype specific epitopes, a set of 50 peptides spanning the amino acid sequences of the HA protein ectodomain of pandemic A/H1N1 2009 (H1N1pdm) influenza virus strain A/ California/04/2009 were synthesized. Each peptide is 15 amino acids in length with five residues overlapping with the adjacent peptides [46] (Fig. 1) . The peptides selected for immunization experiments are shown in Table 1 . These peptides were conjugated with a carrier protein, the keyhole limpet hemocyanin (KLH; Sigma, St. Louis, MO), to improve their immunogenicity [47] . As the water solubility of peptides P5, P15, P16 and P31 were too low to conjugate with KLH directly, these peptides were first linked to 6-aminocaproic acid and then to the tripeptide KKC prior to being conjugated with KLH [48] . In addition, a family of short peptide homologs to the P5 peptide was also synthesized to fine map the epitope contained in the P5 peptide (Fig. 5) . All of the peptides and their conjugates used in this study were synthesized by Sangon (Shanghai) Biotechnol Co., Ltd. (Shanghai, China). Each peptide was purified to homogeneity (.95% purity) by highperformance liquid chromatography and verified by mass spectrometry.