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Summary The following experiments have been carried out: (1) enzymatic digestion with pronase and RNase, to study the nature of the cytoplasmic inclusions (viroplasm and cytoplasmic tubules) induced by the virus, and of the smaller virus particles as seen in the viroplasm; (2) autoradiographic experiments after [H3]-uridine incorporation, to detect the possible sites of virus synthesis and/or assembly. Enzymatic digestion of maize leaf tumours caused by maize rough dwarf virus (MRDV) showed that the viroplasm and cytoplasmic tubules induced by the virus are composed mainly of protein, and that the small particles embedded in the viroplasm are ‘naked’ RNA particles, since they were promptly digested by RNase, while the complete particles were not. Autoradiographic experiments demonstrated that [H3]-uridine was incorporated almost exclusively into the viroplasm containing virus RNA particles, while no incorporation was detected in the viroplasmic regions free of virus particles and in the mature virus particles as seen in the cytoplasm. It is concluded that the viroplasm is most probably the site of assembly of MRDV.