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DNA showed a 20 per cent decrease in the incorporation of labelled UMP while under the conditions for the RNAprimed reaction there was a slight increase in incorporation.No apparent differences were observed in the activities of DNA nucleotidyltransferase or DNA-primed RNA nucleotidyltransferase in preparations from infected and uninfected cells and cell fractions (Fig. 1a, b). However, significant increases in RNA-primed RNA nucleotidyltransferase activity were observed in each of the five experiments of this type performed beginning at intervals of 1-3 h after infection (Fig. 1c). The increases were more pronounced in enzyme preparations from nuclear than cytoplasmic fractions.Reich, Franklin, Shatkin and Tatum• have demonstrated that more than 90 per cent of DNA-primed RNA synthesis can be inhibited by actinomycin D in L cells without significant interference with the production of an infectious RNA virus. More recently, Baltimore and Franklin 7 have found that after infection of L cells with mengovirus there is increased incorporation of ribonucleotide residues into polyribonucleotides by a particulate fraction from the cell cytoplasm. These pieces of evidence, together with the finding that RNA-primed RNA nucleotidyltransferase activity is increasod in Krebs II ascites tumour cells on infection with encophalomyocarditis virus, suggest that the rtplication of RNA viruses, at least in some mammalian systems, is dependent on RNA-primed rather than on DNA-primed synthesis of RNA.