The highly pathogenic (HP) avian influenza isolate, A/Fowl/Victoria/76 (H7N7), contains two naturally occurring hemagglutinin (HA) variants. The two hemagglutinin proteins differ only in the possession of a potential asparagine-linked glycosylation site at amino acid position 188-190, which is near the proposed receptor binding region of the HA. Expanded virus plaques which possess the addition site exhibit more slowly migrating HA1 subunits and are significantly more lethal in chickens than those which lack the site. When artificial mixtures of the two variants were inoculated in birds, as few as 1 in 1000 particles containing the glycosylation site was sufficient to exhibit 100% lethality in birds. The data raise the possibility that presence of carbohydrate near the receptor site on the H7 avian influenza virus hemagglutinin may influence virulence.