Antibodies (immunoglobulin G (IgGs)) from antisera raised against viral particles that had been dissociated by treatment with SDS and intact particles of Rice dwarf virus (RDV) were studied for their ability to prevent viral infection of vector cells in monolayers in vitro. Even though IgGs raised against dissociated virus had a higher titer than those raised against intact viruses in an analysis of viral proteins on Western blots, they did not neutralize RDV. Conversely, IgGs raised against intact RDV effectively neutralized viral infectivity. Electron microscopic observation of the aggregation of RDV particles after incubation with IgGs raised against intact RDV, but no aggregation of RDV particles after incubation with IgGs raised against dissociated RDV suggested that IgGs raised against intact viruses might prevent viral invasion by causing clumping of viruses, thereby reducing the number of infectious units. Our results reveal, for the first time, a possible mechanism for the neutralization, mediated by antibodies, of plant viruses that propagate in insect vector cells.