The release of influenza virus particles after seperation of the virus and infected cell membrane is an active process. During the budding process, HA on the surface of the newly budding virion binds to cell surface molecules containing sialic acid residues as seen during attachment. The NA glycoproteins neuraminidase activity is essential to cleave the link between the HA and sialic acid on the surface of the host cell from which the budding virus is emeging from. Thus the NA mediated cleavage of sialic acid residues terminally linked to glycoproteins and glycolipids is the final step in releasing the virus particle from the host cell. This essential role of NA in release of virus particle has been demonstrated with the use of NA inhibitors (Palese, 1976; Luo, 1999; Garman, 2004), ts NA mutant viruses (Palese, 1974) and with viruses lacking NA activity (Liu, 1995). In all cases, viruses remain bound to the cell surface in clumps in the absence of NA enzymatic activity, resulting in loss of infectivity. Addition of exogenous sialidase results in virus release and recovery of infectivity. The sialidase activity of the NA is also important for removing sialic acid from the HA on virus particles, if this is not removed, virus particles aggregate.