In the presence of methyl methanesulfonate (MMS) and N-methyl-N'-nitro-N-nitroso-guanidine (MNNG), the survival of the cells harboring ada or alkA genes on plasmids has been measured. PJ1 cells, ada mutants those are sensitive to the exposure of methylating agents, showed the more increased sensitivity to MMS when they carried the ada gene on plasmid. The ada gene on plasmid, however, did not affect the sensitivity of MS23 cells which are alkA and another sensitive mutants to methylating agents. In case of wild type AB1157 cells, the sensitization was also observed when the cells contain the plasmid carrying ada gene. Enzyme assay showed the level of AlkA enzyme activities in the sensitized PJ1 and AB1157 cells were very high. Thus, it might be reasonable to suggest that the over-production of AlkA enzyme is responsible for the sensitization. We also found that the sensitization of cells was not detected when MNNG was used as the DNA damaging agent. This implies that the sensitization of cells needs specific interactions between AlkA enzyme and the DNA damages caused by MMS not by MNNG.