Brome mosaic virus (BMV) can be radioactively labelled by allowing virus synthesis to take place in barley leaves whose cut ends are placed in solutions of radio active precursors of BMV. When barley plants are infected by inoculation of the first emerging leaf, the rate of incorporation of both [14C]-leucine and [32P]-ortho-phosphate into virus particles was found to be maximal in the second leaf, cut and labelled 4 to 5 days after inoculation. The rate of incorporation declined when the labelling or the cutting and labelling were delayed, even though assembly of particles still took place from pre-formed components. An exposure of 72 h to an appropriate amount of 32P yielded highly radioactive BMV RNA for possible sequencing studies. A specific radioactivity (32P) of the order of 1 mCi/mg RNA can be obtained readily.