A transient decaribonucleotide (iRNA) is covalently linked to nascent eukaryotic DNA chains at their 5′ end. Searching for the putative iRNA polymerase (primase), we detected in extracts from SV40-infected cells a DNA-dependent incorporation of UMP residues from UTP into free and DNA linked deca- or similarly sized ribonucleotidea. Denatured salmon sperm DNA served as the standard template in this reaction. SV40 FIII DNA was also an effective template , SV40 FII DNA was ineffective while FT yielded mainly free decaribonucleotides. The incorporation depended on the other rNTPs and was resistant to high concentrations of α-amanitin and rifamycin AF/013, drugs inhibitory to RNA polymerases I, II and III. The results implicate the decaribonucleotide polymerase in the priming of nascent DNA chains and suggest that the unique size of iRNA is encoded within its primase.