The ability of the HA antibodies from immunized ferrets to neutralize H5N1 virus was analyzed with three assays: a pseudotyped lentiviral vector neutralizing assay, an HA inhibition assay, and a microneutralization assay (Table 1 ; Ferrets). Only HA-containing groups showed substantial neutralizing antibody titer responses, while NP, M2, and NP+M2 groups showed no neutralization in each assay.