We have prepared plasmid pLIOP3, a derivative of λdv whose pRoR-tof region was replaced by lacPO (lactose promoter-operator)-lacI (repressor for the lac operon). pLIOP3 did not transform Escherichia coli polA. A mutant of pLIOP3 which could multiply as a plasmid was isolated after hydroxylamine mutagenesis. The mutant had two base substitutions in lacPO, which made the operator constitutive. The mutant could become transformed even after the removal of the replication origin of pBR322 which was contained in pLIOP3. The mutant which contains only the replication origin of λ is named pLmI3. The synthesis of pLmI3 DNA was stimulated about 10-fold by the addition of an inducer of lactose promoter and the DNA recovered after induction had the same structure and biological activity as that recovered before induction. Cells carrying pLmI3 were killed. by the induction. The replication of pLmI3 DNA depended on O and P gene products and the "ori" region of λdv.