Summary A procedure for the purification of rubella virus from infected suspensions of BHK 21 cells resulted in preparations containing 1 to 3 × 1010 p.f.u./ml. In sucrose gradient centrifugation the area of maximal infectivity (buoyant density of 1.175 g./ml.) coincided with peaks in haemagglutinating activity, and in E 260. It also coincided with presence of virus-like particles, and a sharp band visible by naked eye. Specific activities of the order 30 × 106 p.f.u./µg. protein, 106 HAU/µg. protein, and 5 to 10 × 105 p.f.u./HAU were achieved. Glutaraldehyde-fixed negatively stained rubella virus preparations showed round particles with rough surfaces measuring 50 to 73 nm. (average 61 nm.) in diameter. Unfixed viruses had a greater size variation, 55 to 89 nm. (average 74 nm.) in diameter, apparently due to deformability and fragility of the particles. Spontaneous and deoxycholate-induced breakdown of the particles showed rupture of the envelope but revealed no characteristic inner structure.