The 14S mRNA from MOPC 173 tumour has been transcribed into cDNA by AMV DNA polymerase and converted into a double stranded form by Escherichia coli DNA polymerase I. After addition of oligo-dG tracts at the 3'-OH ends by calf thymus terminal transferase this DNA was hybridized to an E. coli plasmid (pCR1) to which oligo-dG tracts had been similarly added. Circular molecules resulting from GC base pairing have been used to transform C 600 E. coli cells and to confer kanamycine resistance. Several recombinants have been obtained containing the V+C regions and the C or V region alone. These recombinant molecules are being used to analyse the translocation of V and C genes and to purify V and C genes from DNA of germ line cells and of differentiated tumour.