SUMMARY: Preparations of the Rothamsted tobacco necrosis virus were made by the ultracentrifugation of sap from infected tobacco leaves after a preliminary concentration by freezing. Not all the anomalous nucleoprotein in these preparations was infective, and the products were fractionated by differential ultracentrifugation at lower speeds and by precipitation at pH 4 in the presence of sedimentable protein from uninfected leaves. The more readily sedimentable and precipitable material carried with it most infectivity, whereas the other material had the greater sero-logical activity.

Preparations made quickly from freshly expressed sap were less infective than those made from sap that had been frozen or allowed to age for a few days. The extent of the activation produced by these treatments depended on the physiological condition of the infected leaves.

As much virus could be extracted from the leaf residues as occurred in the sap. The infectivity of this residual virus depended on the medium used for its extraction.

It is suggested that much of the infectivity of this virus in sap is acquired during or after extraction from the leaf, but the relationship between the particles with different sizes and properties remains uncertain.


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