Lectin staining and H4N6 influenza virus infectivity after NA depletion of cell surface sialic acid are shown. Removal of sialic acids from the surfaces of SRECs (A) and HRECs (B). Cells were treated with either an α2,3-specific NA or an α2,3/α2,6 NA, followed by cell surface lectin staining with MAA (SAα2,3Gal) or SNA (SAα2,6Gal). Values shown are the geometric means of fluorescent intensity (± standard errors of the means [SEM]) normalized such that the fluorescence intensity of untreated cells is 100% and the fluorescence intensity of mock lectin-treated cells is 0%. Data are the results of four separate experiments performed in duplicate. Virus infectivity levels after NA treatment of SRECs (C) and HRECs (D) are shown. Data are the results of four separate experiments with each virus assayed in triplicate for each experiment. Results are the means ± SEM. Statistically significant differences were observed between the infectivity levels of untreated and α2,3/α2,6 NA-treated cells (analysis of variance-protected Student's t tests; *, P < 0.05; **, P < 0.01).