A method is described to clone cDNAs corresponding to the 5′ end of specific mRNAs which are absent from cDNA libraries. A segment is excised from the 5′ end of a previously cloned truncated cDNA and hybridized to total polyadenylated RNA. The hybridized primer is extended using reverse transcriptase, tailed with deoxycytidine, and the second strand synthesized using oligo (dG) 10–16 as a primer. The primer-extended double-stranded cDNA is cloned into pBR322 by dC–dG homopolymeric tailing. This method has been successful for cloning a 398 bp cDNA fragment corresponding to the entire 5′ end of rat pancreatic elastase I mRNA, a prominent pancreatic mRNA, and a 510 bp fragment encoding the 5′ end of kallikrein mRNA, a lower abundant pancreatic mRNA.