Summary The nucleic acid of rubella virus was shown to be RNA by the incorporation of [3H]-uridine into virus particles, and by the presence of ribonuclease-sensitive material, sedimenting mainly at 38 to 40 s, in sodium dodecyl sulphate extracts of purified virus. There were at least two virus-specific RNAs in cytoplasmic extracts of BHK 21 cells infected with rubella virus. One, possibly the structural RNA of the virus, was sensitive to ribonuclease and sedimented at 38 to 40 s like the RNA extracted from virus particles. The other had a heterogeneous sedimentation pattern with main peak at 20 s. It was partially resistant to ribonuclease and might be the replicative intermediate form of the RNA of rubella virus.