The antigenic properties of the tobacco mosaic virus (TMV) have been studied extensively for more than 50 years. Distinct antigenic determinants called neotopes and cryptotopes have been identified at the surface of intact virions and dissociated coat protein subunits, respectively, indicating that the quaternary structure of the virus influences the antigenic properties. A correlation has been found to exist between the location of seven to ten residue–long continuous epitopes in the TMV coat protein and the degree of segmental mobility along the polypeptide chain. Immunoelectron microscopy, using antibodies specific for the bottom surface of the protein subunit, showed that these antibodies reacted with both ends of the stacked disk aggregates of viral protein. This finding indicates that the stacked disks are bipolar and cannot be converted directly into helical viral rods as has been previously assumed. TMV epitopes have been mapped at the surface of coat protein subunits using biosensor technology. The ability of certain monoclonal antibodies to block the co–translational disassembly of virions during the infection process was found to be linked to the precise location of their complementary epitopes and not to their binding affinity. Such blocking antibodies, which act by sterically preventing the interaction between virions and ribosomes may, when expressed in plants, be useful for controlling virus infection.