Glycans terminating in sialic acids serve as receptors for influenza viruses. In this study ferrets were infected with influenza virus A/New Caledonia/20/99, and the in situ localization of sialic acids linked a2-3 and a2-6 in the airways was investigated in infected and non-infected animals by use of sialic acid detecting lectins and a monoclonal antibody towards the Sialyl-Tn antigen. The goblet cells in the bronchi from non-infected ferrets expressed Sialyla2-6Gal glycans, while the seromucinous glands in the submucosa expressed Sialyla2-3Gal glycans. In the infected animals, the surface epithelial cells in some bronchi showed metaplasia and expressed the Sialyl-Tn antigen: Sialyla2-6GalNAc-O-Thr/Ser. The submucosal tracheal glands in these animals showed increased expression of both Sialyla2-3 and Sialyla2-6 epitopes.