The 12S units were purified by heat treatment and acidification of purified, highly-concentrated virus, with separation of viral RNA by gel filtration. The following physical parameters were obtained: - partial specific volume 0.737 +/- 0.020 g/cm3; diffusion soefficient D020,W = 3.96 +/- 0.24 F; sedimentation coefficient S020,W = 12.1 +/- 0.5S; molecular weight 283,00 +/- 30,000 Dalton; refraction increment (determined by ultracentrifugation) was dn/dc = 0.189 +/- 0.010 cm3/g for white light of the "HBO 200". Examination of the 12S component by analytical ultracentrifugation and in sucrose and CsCl gradients showed that the product was uniform; density determined in CsCl was 1.302 +/- 0,00 s g/cm3. The ultraviolet absorption spectrum was characteristic of proteins; extinction values at 278 nm in a layer 1 cm thick and 10 mg/ml at pH was 13.9 +/- 1.9, while the ratio E278 nm/E250 = 2.2. Electron microscopy showed that the diameter was 11.0 +/- 1.6 nm. Morphology of 12S unit was discussed.