H9N2 influenza A viruses are currently widespread in chickens, quail, and other poultry in Asia and have caused a few cases of influenza in humans. In this study, we found that H9N2 viruses from Hong Kong live bird markets have receptor specificity similar to that of human H3N2 viruses. In addition, the neuraminidase of poultry H9N2 viruses has mutations in its hemadsorbing site, a characteristic resembling that of human H2N2 and H3N2 viruses but differing from that of other avian viruses. Peculiar features of surface giycoproteins of HgN2 viruses from Hong Kong suggest an enhanced propensity for introduction into humans and emphasize the importance of poultry in the zoonotic transmission of influenza viruses. © 2001 Academic Press.