The DNA replication inhibitor peptide microcin B17 is shown to be a useful tool for preparing Escherichia coli maxicells. To illustrate its usefulness, we have identified polypeptides synthesized from pMccB17 and R100 IncFII miniplasmids. After comparing the respective polypeptides and the miniplasmid restriction maps, we concluded that these plasmids share extensive homology in the basic replicon but are different for an adjacent region (parD) that is involved in plasmid stability and maintenance.