‘Strong-stop' DNA, complementary to the 5′-terminal 101 nucleotides of Rous sarcoma virus genomic RNA and subgenomic mRNAs can be used to isolate RSV-specific mRNA from infected cells. To analyse leader sequences of these mRNAs and to study the mechanism of their translation, relatively large quantities of strong-stop DNA are required. The construction of four plasmids carrying useful strong-stop DNA sequences for mRNA isolation is described. One such hybrid plasmid carries a DNA insertion containing sequences derived from a region starting 42 nucleotides from the 5′-end of RSV RNA, and ending at the tRNATry primer attachment site. Since this plasmid lacks the 20-nucleotide terminal repeat sequence it can be used for the specific isolation of 5′-end fragments of RSV-specific mRNAs and of complementary DNA transcripts of these fragments.