Abstract: Objective To study the attachment and subsequent entry of H5N1 virus to A549 cells and Hep-2 cells. Methods Expression of SAa2,3Gal and SAa2,6Gal on A549 cells and Hep-2 cells was examined by using lectin fluorescence and flow cytometry. H5N1 virus entry into A549 and Hep-2 cells was measured by indirect immuno?uorescence assay. The entry efficiency was determined by Western blot analysis. Results SAa2,3Gal was prevalent in A549 cells and Hep-2 cells, while SAa2,6 Gal was little found. Furthermore, SAa2,3Gal expression was more regularly observed in Hep-2 cells, rather than A549 cells. The H5N1 virus tested could enter A549 cells and Hep-2 cells. However, viral entry efficiency differed between the two cell lines tested. A549 cells were found to be more susceptible to avian influenza than Hep-2 cells. H5N1-induced cell death was inefficient in A549 cells than Hep-2 cells. Conclusion The expression of SAa2,3Gal on the cells tested corresponded with the attachment of the H5N1 virus. However, sialic acid only may not sufficient for entry into cells.