Publisher Summary The chapter discusses on the antigenic structure of tobacco mosaic virus (TMV). Compared with other antigens, TMV is exceedingly large, yet not so large that the virus cannot be treated as if it were a soluble protein. The serological reactions of TMV are characteristic of those antigens, whose size places them between the small protein molecules, on one hand, and bacteria or cells, on the other. Thus, interactions of TMV with antibodies are somewhat unique and reveal a number of problems not apparent with either the smaller or larger antigens. For these reasons, the TMV anti-TMV reaction has attracted the attention of immunologists as well as virologists. TMV may dispense with its protein entirely, either as a result of chemical manipulation or mutation, and still retain its infectivity. But when protein is combined with the nucleic acid, it apparently plays some role in establishing the infection. At present, that role is unknown. As far as inactivation with antiserum is concerned, it is known; however, that antibody to the coat protein is without effect on the nucleic acid. Even after the whole virus is inactivated by antibody, the RNA within the rod retains its infectivity, for it can be extracted with phenol from the inactive precipitate with its original capacity for infection undiminished.