Cucumber green mottle mosaic virus (CGMMV) is a rod-shaped virus closely related to tobacco mosaic virus (TMV) in its chemical and immunological characteristics, and also identical in size. Both viruses show an extremely similar tertiary structure in the ordered gels. The coat protein of both viruses has about 37% sequence homology. Despite such a remarkable resemblance between their structures of both viruses and proteins, and between the schemes of the constructions of both viruses, they have fairly different biological activity. When the two constituents of the virus, coat protein and RNA, are cultivated in a greenhouse at the temperature above 30^oC, the yield of TMV is much better than CGMMV, and moreover, the former is known to have intensive infectivity than the latter. The question remains, therefore, open why both viruses have such different infectivity. In the present paper, the problems about the structural differences of subunit monomer, different self-assembling process of coat protein, local structural difference of RNA, different reconstitution process of coat protein and RNA from both viruses, different penetration behavior of coat proteins into model membrane are discussed. From these results the antiviral activity of polysaccharides against infection of TMV are then discussed because the large aggregates of TMV in the presence of polysaccharides prevent the penetration of TMV-RNA into the cell membrane and/or the blocking the decapsulation process of TMV coat protein at the earliest infection stage.