Serum samples were collected from 11 convalescent patients during the early 2009 H1N1 pandemic in Beijing. The diagnostic criteria for H1N1 influenza virus infection of these patients fully followed the WHO's descriptions. Sera from 10 healthy blood donors were used as negative controls. In addition, serum samples collected from 100 blood donors were recruited to evaluate the peptide-ELISA assay developed in this study. All these samples were kindly provided by the Beijing Municipal Center for Disease Control and Prevention (Beijing, China) and written informed consent was obtained from all participants. All samples were coded prior to analysis to ensure anonymity and the procedures were approved by the Institutional Medical Ethic Review Board of the Institute of Pathogen Biology, Chinese Academy of Medical Sciences (Beijing, China).