SUMMARY: Tobacco rattle virus was isolated from tobacco sap by differential centrifugation; the yield was about 50 mg. virus/1. sap. Purified preparations were highly infective, precipitated optimally at pH 4·0–4·5, and contained one electrophoretic component; chemical analysis and the ultraviolet absorption spectrum suggest they contain about 5% nucleic acid and 95% protein. Most rod-shaped particles in purified preparations were 73–77 mμ or 179–192 mμ long: all were 25 mμ wide. These two components were separated by rate zonal centrifugation in sucrose density-gradient solutions: both have the same density in solution as tobacco mosaic virus, and they have sedimentation constants of about 198s and 295s, respectively. No differences were found between long and short particles in serological behaviour, electrophoretic mobility or ultraviolet absorption spectrum; apparently they differ little in gross chemical composition. Only the long particles are infective. Particles slightly shorter than 179 mμ seem non-infective; some of these may be aggregates of two short ones. Short particles rarely aggregate end-to-end at pH 6·8–7·0 but aggregation increases as the pH value decreases.569


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