Abstract
Ultrastructural investigations of different hosts infected with the tombusvirus carnation Italian ringspot have shown that the multivesicular bodies typical of tombusvirus infections develop from modified mitochondria. Digestion with RNase in high- or low-salt medium demonstrated that multivesicular bodies contain RNA, some of which is double-stranded and, possibly, of viral origin. Cytochrome oxidase was identified in multivesicular bodies by cytochemical tests.