TOBACCO necrosis is a disease that can be caused by several serologically unrelated viruses1, all of which are soil-borne and cause necrotic local lesions in leaves of tobacco (Nicotiana tabacum L.) and French bean (Phaseolus vulgaris L.) plants. The virus Bawden and Pirie2,3 called the ‘Rothamsted culture’ differed in several respects from other members of the group ; the ratio of infectivity to specific antigen content varied greatly in different preparations, it lost infectivity in vitro more rapidly than the others, and infective preparations always contained particles of two different sizes, about 17 mµ and about 30 mµ in diameter. Crystalline preparations (identified as nucleoproteins), consisting almost entirely of the smaller particles, had little or no infectivity, whereas other partly purified preparations that contained many more of the larger particles were infective. From this, Bawden and Pirie concluded that probably only the larger particles were infective and that the smaller ones, which were usually the major constituent of their preparations, were an additional product of the multiplication of the larger ones4.