Deletions or insertions in the copB gene of plasmid R1 result in a copy mutant phenotype. The wild-type copB gene has been cloned on various plasmid vectors. The presence of such chimeric plasmids reduced the copy number of R1 copB mutant plasmids to normal or subnormal levels, indicating the expression of a trans-acting inhibitor activity from the copB chimeras. However, the cloned copB gene did not affect the copy number of wild-type R1, and no incompatibility was exerted by the cloned copB gene against wild-type R1 (or R100). Although the copB gene is not normally required for the incompatibility exerted by copA, it is shown that the CopB function is required for expression of incompatibility by the copA gene from some types of chimeric plasmids. Mutant plasmids that have lost both Cop functions replicate in an uncontrolled fashion.