Although colicin E1 type plasmids (colE1 and pKY-1) can not transform Escherichia coli recB recC sbcB, pBR322, a derivative of a colicin E1 type plasmid, can transform it. The analysis by using the derivatives of pKY-1 gave the following conclusions concerning their inability. 1) E. coli recB recC sbcB was not transformed with the plasmid carrying cea gene of colicin E1 which was transcribed by constitutive promoter. 2) Transcription of the first 250 bases of the cea gene was sufficient to inhibit the transformation. 3) The inhibition of transformation with the plasmid is expressed by transcription of its own cea gene (cis-acting). 4) When the replication origin of pKY-1 was changed to pKYM, the transformation was not inhibited. The transcript inhibits only the transformation with the plasmid carrying the replication origin derived from colicin E1 or its relative. 5) Transcription of the cea gene inhibited multiplication of the host cells carrying the plasmid. The synthesis of plasmid deoxyribonucleic acid (DNA) continued in the host cells but the DNA was not isolated from the cytoplasm as a closed circular form.