The use of x-ray inactivation of virus particles is suggested as a means of obtaining an estimate of the size of the ultimate entity inactivated that is independent of any inert material on which it may be absorbed, an objection inherent in methods dependent on filtration and rate of fall. Ordinary tobacco mosaic virus and several of its derivatives were irradiated with x-rays from 3 metals, Cr, Cu and Ag, having effective wave lengths of 2.1, 1.5 and 0.7 A. The inactivation rates of the several virus strains were essentially alike; all showed a wave-length effect which is explicable on the hypothesis that one absorption of x-ray energy within the portion of the particle essential for reproduction is sufficient to inactivate. The computed values corresponding to the absorption data are: ave. size of "vital volume" of mosaic particle, 7.5 X 10-18 cm3; atomic volume, 940,000; molecular wt. 15,000,000. The latter is in essential agreement with detns. by other methods.