Interaction of antiviral antibody with the virion can result in the formation of infectious VA complexes. These complexes have been recovered from the blood of chronically infected animals and have been produced in vitro. Incubation of infectious VA complexes with specific anti-immunoglobulins or the purified first and fourth components of complement resulted in neutralization. With subneutralizing concentrations of the first and fourth components of complement, neutralization was enhanced by the addition of the second and third components of complement. RF attached to infectious VA complexes but failed to produce neutralization. The attachment of RF to VA complexes, however, increased the susceptibility of these complexes to neutralization by complement. These findings support the hypothesis that anti-immunoglobulins and complement result in neutralization by more extensive coverage of the surface of the virion than occurs with antiviral antibody alone. Other experiments showed that antiviral antibody increased the sedimentation of radio-labeled virus and that rate-zonal centrifugation could be used to purify VA complexes and to study factors which influence the association and dissociation of these complexes. Our experiments suggest that in vivo the attachment of accessory factors to infectious VA complexes might enhance virus neutralization and play a role in the pathogenesis of virus-induced immune complex disease.