The mutagenesis-enhancing plasmid pKM101 and its mutant pSL4 were introduced into Escherichia coli B/r strains possessing different DNA repair capacities ($phr^{-}, recA^{-}, uvrA^{-}, uvrB^{-}$) and determined the protection effect and mutagenecity for UV and MNNG. The mutability and protection effect of plasmid pKM101 and pSL4 were affected by different DNA repair capacity. The mutagenecity and resistance of two plasmids were increased against UV and MNNG, and plasmid pSL4 had a higher effect than pKM101. We suggest that the functional differences between pKM101 and pSL4 is due to the variety of mutator gene.