Different samples of purified tobacco mosaic virus show a relatively wide variation in solubility in ammonium sulfate solution. This variation and the type of solubility curve obtained in the presence of varying amounts of solid phase show that the purified virus whether isolated by mild treatment with ammonium sulfate or by ultracentrifugation is not a homogeneous chemical substance but contains more soluble and less soluble virus fractions of comparable specific activities. Long contact with strong ammonium sulfate solutions or 0.1 M phosphate buffer results in a decrease in solubility. The variation in the solubility of samples isolated from different plants by the same method seems to depend in part on the length of time the plants are inoculated before they are cut, and probably also on the conditions under which they are grown. Virus preparations isolated from plants of different genera grown under the same conditions and inoculated at the same time, however, behaved like identical substances in solubility experiments.

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