The resolution system from in 1000 was cloned into a pBR322 recombinant plasmid carrying E. coli threonine operon. The resulting plasmid displayed a significant increase in segregational stability with respect to the original plasmid, either in a rec(+) or a rec(-) E. coli strain (100% of plasmid-containing cells versus 64% and 1% respectively). The Tn 1000 resolution system proved to be responsible for the stabilising effect exerted by the transposon when present in recombinant plasmids.