A host-cell-selected variant of influenza B virus with a single nucleotide substitution in HA affecting a potential glycosylation site was attenuated in virulence for volunteers.

In the present paper we describe marked differences in the virulence for man of influenza B viruses isolated from the same clinical specimens but cultivated in mammalian cells (human trachea) or in eggs. The presence of a single glycosylation site located near the tip of the HA molecule at residues 196-198 appears to covary with virulence.