In conclusion, our data provide evidence that the H1 subtype HA harbors more epitopes than what has been found previously. The conservation of an epitope (P5, aa 58-72) in the H1-subtype HA of IAV and its near complete absence in other subtypes indicate that this epitope meets the critical requirements for diagnosis of H1 subtype influenza virus infections. The peptide-ELISA developed in our study may be applicable for serodiagnosis and may serve as a universal diagnostic tool for H1subtype IAV surveillance.