A standard procedure for the isolation of virus specific double-stranded RNA from cowpea mosaic virus-infected Vigna leaves is described. The double stranded RNA is characterized by its buoyant density in caesium sulphate and its melting temperature (Tin). The buoyant density and the Tm are compared with those of other double-stranded RNA's on basis of their guanine + cytosine content. The frequency distribution of the lengths of the molecules, determined by electron microscopy, indicate the

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... urrence of double-stranded RNA molecules specific for each of the two RNA's of cowpea mosaic virus. During hybridization with labelled cowpea mosaic virus RNA, the virus RNA is specifically incorporated into the double-stranded structure.