The length of B cell epitopes can vary from 5 to 20 amino acids [45, 46] . To map the epitope contained in P5, we performed a peptide-inhibition ELISA using a series of N-terminal and Cterminal truncated peptides. However, we found that the fulllength P5 (15 aa in length) rather than truncated peptides showed strongest binding to the corresponding antibody ( Fig. 5 and Fig. 6 ). As peptides N10 and C12 are the shortest truncated P5 that can bind to anti-P5 antibody and share a core sequence of LRGVAPL, we tested whether this sequence could be the epitope. However, the synthetic peptide LRGVAPL did not block P5-antibody interactions nor bind P5 antibody (Fig. 6) . Thus, we speculate that adjacent amino acids to this sequence are also involved in the binding of antibody elicited by P5-KLH conjugates. The concept of using linear epitopes in influenza virus diagnostics and control has not been extensively investigated. In a recent study, an epitope-blocking ELISA, which can universally detect antibodies to human H5N1 virus, has been developed [17] . Our results show that a peptide-ELISA based on the highly conserved H1 subtype-specific epitope can also be used for the detection of H1 antibodies, displaying good correlativity with the HI test. Our results indicate the potential of the P5 epitope in H1subtype IAV diagnosis. However, the performance of this assay needs to be further evaluated in studies with large scale samples.