Virus entities associated with necrotic ring spot, yellows, and green ring mottle of sour cherry, tatter leaf of sweet cherry, prune dwarf, and two viroses of rose have been purified and concentrated from cucumber leaves and/or peach and cherry petals and peach leaves by differential centrifugation. To prevent inactivation of the virus, the Prunus tissues were homogenized with 0.1% potassium cyanide in 2.5% sodium bisulphite. Infective extracts of some of the entities were also prepared by ammonium sulphate precipitation and from aqueous chloroform–amyl-alcohol emulsion. Particles ranging from less than 10 to more than 100 mμ in diameter were observed with the electron microscope in extracts from both healthy and virus-infected sources. In the former, prepared by differential centrifugation, particles were distributed about a series of peaks at 10 mμ intervals and, in the latter, about a single mode characteristic of the entity under examination. Particles associated with different entities ranged from 28 mμ diameter for rose mosaic to 50 mμ for some tatter leaf and green ring mottle isolates. In extracts known to contain two viruses, characteristic distributions of each component of the mixture could be determined if the mean particle diameter of one virus was known and differed from that of the other entity by 5 mμ or more. Evidence based on particle characteristics, supplemented by cross protections and comparative symptomatology suggests that the necrotic ring spot virus is not necessarily involved in the etiology of cherry yellows, prune dwarf, tatter leaf, or green ring mottle.