We have prepared three kinds of plasmids containing two lactose promoters in palindromic positions. In the first, structure I, the promoters are joined tail to tail and transcription proceeds from head to tail. In structure II they are joined head to head. In structure III a foreign DNA is located between the two lactose promoters. The plasmid of structure I can not multiply in wild-type Escherichia coli but can in E. coli recB recC sbcB recF. Since a plasmid with structure III can grow in the wild-type strain, a linear DNA fragment which is prepared from structure III by removing the foreign DNA can be used as a very efficient cloning vector. Plasmid DNA with structures I and II isolated from E. coli recB recC sbcB recF were shown to have a relaxed form by agarose gel electrophoresis. However, in contrast to the structure I plasmid, structure II plasmid can multiply in wild-type cells, though it is unstable and present in a low copy number. Excised head to head dimer, but not tail to tail dimer, moved at an unusual rate in polyacrylamide gel electrophoresis. The difference between the physical structures of the two dimers of the lactose promoter may be reflected in the biological character of the plasmids.