The effects of exogenous RNA on the poly(A).oligo(dT)-dependent DNA-polymerase activity of factor and factor-free carcinomas of the mammary gland of C3H mice and Ehrlich ascite carcinoma were studied. It was shown that after intraparenteral injection of the RNA the enzyme activity is increased in the oncornavirus-containing carcinomas and remains unchanged in the virus-free tumours. The effect of RNA is not due to the glucocorticoid release into the blood and is suppressed by actinomycin D one hour prior to the addition of RNA. The increase of the poly(A).oligo(dT)-dependent DNA-polymerase activity of the cytoplasmic fraction of tumour cells is correlated with the rise of cytoplasmic RNA-polymerase activity and the increased incorporation of the precursors into the cytoplasmic RNA. Thus, in virus-containing tumours exogenous RNA induces the synthesis of cytoplasmic RNAs responsible for the synthesis of RNA-dependent DNA-polymerase.