Virus binding assays with swine H1N1 and avian H2N3 viruses on porcine alveolar and tracheal serial sections are consistent with virus affinity for particular host receptor type. The presence of both SAα2,6-Gal and SAα2,3-Gal receptors in alveoli is mirrored by a similar overlapping binding pattern of swine and avian viruses to pneumocytes. By contrast, the dominant SAα(2,6)-Gal receptor type on tracheal epithelium shows preferential binding of swine H1N1 virus. Control sections are without virus treatment; low level of auto-fluorescence was detected on alveolar section. 1. alveolar pneumocyte, 2. epithelial lining. Scale bar = 50 μm.