A 4s RNA species, known as Spot 1 RNA, has been identified in RSV (ROW Sarcoma Virus) by bidimensional polyacrylamide gel electrophoresis [ 1 ] . It represents the most plentiful form of 4s RNA, and also the form which is most strongly associated with the 70s RNA genome of RSV [ 1,2] . This RNA has been identified as tRNAtrp by aminoacylation and by determination of its primary structure [3,4]. Experiments involving tagging with CX-~~ P-dATP [2,5] , heat denaturation of the RNA genome [2,5], reassociation of Spot 1 RNA to the 30-40s RNA subunits [6,7] and more recently inhibition by aminoacylation with tryptophan of the loss of priming activity after periodic acid oxydation [8] have provided evidence for its role as a primer of the in vitro transcription of the genome of two avian oncornaviruses, RSV and AMV (Avian Myeloblastosis Virus). In order to determine whether Spot I RNA may act as a primer [9] in the transcription of the murine