Purified preparations of influenza B/Hong Kong/5/72 virus were characterized by hydrodynamic measurements, EM and small-angle neutron scattering. As judged by these techniques, the preparations are highly monodisperse, the virus particles being spherical and of MW .apprx. 200 .times. 106. The lipid bilayer is located at a radius of 425 .ANG. and its MW is estimated to be 60 .times. 106, constituting .apprx. 30% of the total virus mass. The external radius is .apprx. 580 .ANG.