It was found that a plasmid which had a foreign deoxyribonucleic acid (DNA) between two repeated sequences did not multiply in E. coli recBCsbcB, even if it multiplied in wild-type E. coli, E. coli recBC or E. coli recBCsbcBrecF when the insert was longer than 351 base pair. The multiplication of these plasmids were, however, inhibited when a plasmid expressing recF gene was introduced into E. coli recBCsbcBrecF. The inviability of the plasmid carrying the repeated sequence in E. coli recBCsbcB was discussed by the mechanism of recombination, and the functions of recF, recBC and sbcB were speculated. When E. coli recBC was transformed with pDR1 which was a derivative of pBR322 carring a directly repeated sequence between which a DNA fragment derived from plasmid R6K with its origin was inserted, the intramolecular recombinant appeared. The recombinant recovered was, however, only the plasmid which had the replication origin of pBR322. The result suggests that pBR322 is compatible with pDR1 but R6K is not. The replication origin of R6K seems to be preferrentially used by pDR1.