The Escherichia coli ColE1 plasmid, which codes for production of colicin E1, is inherently nontransferable (nonconjugative) by bacterial mating. ColE1 can be transmitted at mating by a process called mobilization if ColE1 is coresident with a transfer plasmid. Mobilization is governed in part by a ColE1 gene called mob. ColE1 is mob+. Several ColE1 derivatives employed in recombinant DNA experiments, notably pBR313 and pBR322, are mob-. These cloning vehicles are mobilized at a markedly reduced frequency relative to ColE1. E. coli K12 carrying either pBR313 or pBR322 represents a useful host vector system for recombinant DNA experiments and affords a significant degree of biological containment.