The previously described method of assaying the antibody response to influenza virus by single-radial-immunodiffusion is based on the formation of opalescent zones when immune sera are allowed to diffuse into gels containing large amount of purified influenza virus. The zones are not thought to be the result of immune aggregates of virus particles and antibody since the virus is immobile in the gel and the average distance between two neighbouring particles is approximately 50 times the virus diameter. The present study investigates how different rabbit IgG-fragments react in this type of test. It was found that IgG, F(ab')2 and reduced (monovalent) IgG gave rise to distinct zones of opalescence. Fab', on the other hand, produced weak and hazy zones which could not be quantitated with any accuracy. We conclude that the most likely mechanism responsible for the zones is a light-scattering effect caused by antibodies attached to the viral surface and that the quality of the opalescence to some extent seems to be dependent on the Fc-fragment.