We report here the identification of antigenic peptides of the 2009 A(H1N1) influenza HA protein from a combinational library of viral protein fragments displayed on the surface of yeast cells and sorted by FACS. The peptide fragments were constructed in a way to have a broad but limited range of lengths [24] , and screened using antisera from mouse, goat and human. We then used a novel statistical approach to identify antigenically important regions of the viral protein based on the frequency of each residue appearing in the antigenic peptides at both the primary sequence and structural level.