In addition to their independent functions, the balance between the binding affinity of the viral HA and the sialidase activity of the NA is also critical for efficient infection. The ability of A/Victoria/ 3/75 viruses with mutations or substitutions in either the HA or NA alone to infect similar numbers of cells and replicate to comparable peak titers as for wild-type virus at 37uC implies that these viruses were not crippled by the mismatch between the specificities of their HA and NA. Replication and spread of influenza viruses that possess an avian HA paired with its ''matched'' NA was even more compromised than that of recombinant viruses with individual changes to levels seen with wholly avian viruses. Thus, viruses with co-evolved glycoprotein pairs exhibit restricted replication at low temperatures and both HA and NA genes contribute to the phenotype.