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An investigation has been made of the optical rotatory dispersion in the region 226 to 366 mmu of tobacco mosaic virus (TMV), the protein subunits isolated therefrom, the rods synthesized from the protein subunits, and the ribonucleic acid (RNA) isolated from TMV. Both TMV and the protein rods show anomalous rotatory dispersion. The RNA shows a Cotton effect with an inflection point around 260 mmu, which is shifted to 272 mmu in concentrated urea solution. A suggested interpretation of the RNA rotatory dispersion is given. The rotatory dispersion of the protein subunits shows an incipient Cotton effect with an inflection point around 293 mmu and the beginnings of a large negative Cotton effect with a trough at 232 mmu. The dispersion data from the protein subunits can be interpreted to indicate that they contain between 25 and 35 per cent alpha-helix. On the basis of recent sequence investigations and the relationship between amino acid composition and polypeptide structure, the helical portion of the protein subunits can be located in the central section of the protein chain.