SV40 DNA I, injected into Xenopus oocyte nuclei is transcribed. The SV40-specific RNA molecules migrate on sucrose gradients as do viral RNAs formed in infected green monkey cells but a variable proportion of RNA sequences complementary to SV40 DNA is also found in the light region of the gradients. All SV40-specific RNA species seem to be synthesized by RNA polymerase B as their synthesis is completely sensitive to low concentrations (0.1 μg/ml) of α-amanitin. Concomittantly, the formation of SV40-specific proteins (tumor antigens) is inhibited by injecting α-amanitin together with the SV40 DNA.