Although it remains a mystery whether the 1918 pandemic virus was transmitted directly from avians or was the result of reassortment before the pandemic, the 1957, 1968 and 2009 pandemic viruses were reassortant viruses, and the 2009 pandemic was confirmed to be of swine origin and was hence named swine-origin H1N1 influenza viruses . Although S-OIV causes mild disease, it has spread worldwide to more than 214 countries and has caused over 18,449 laboratory-confirmed deaths as of August 1, 2010 due to relatively efficient transmission among humans. Swines susceptible to both avian and human influenza viruses and possessing a trachea that expresses both Siaa2-3Gal and Siaa2-6Gal provide direct evidence supporting the theory of pigs as “mixing vessels” for the creation of reassortant viruses. However, receptor binding specificity data have shown that classical swines preferentially recognized NeuAca2-6Gal and that avian-like swine switched their binding preference to NeuAca2-6Gal over time. Analysis of amino acids has suggested that HA mutations are responsible for the increase in affinity of the virus for NeuAca2-6Gal. However, why influenza viruses that have continued replication in pigs evolve HA receptor specificity to NeuAca2-6Gal is still puzzling. Types, structures and distribution of glycans on the host cell surface are thought to be associated with viral HA receptor specificity. Thus, we elucidated the structures of N-glycans required for influenza A infection from the porcine trachea, an early site of influenza virus attack and replication, and porcine lung, a principal swine influenza replication site, by HPLC and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analyses. Our results for double lectin immunostaining agree with previous observations that there is dual expression of Siaa2-6Gal and Siaa2-3Gal receptors on the surface of porcine tracheal and lung epithelia, while Siaa2-6Gal is dominant. Both human and avian influenza viruses have been isolated from pig populations, SB203580 indicating that transmission of whole human or avian viruses to swines has occurred in nature. Although SNA and MAA lectins are useful for detection of the localized distribution of a2-6 and a2-3 linkages, respectively, between Sia and Gal, influenza viruses can distinguish not only glycosidic linkages but also types of Sias, which influence the influenza virus host range. HPLC with MALDI-TOF-MS techniques have been continuously developed over the past years for determination of precise N-glycan structures and quantitative analysis of those N-glycans; influenza virus was reported to bind to but unable to be internalized into cells deficient in N-linked glycan, in spite of no deficiency in O-linked glycan, suggesting the requirement of N-linked glycoprotein for successful endocytosis of influenza virus into its host.