An isogenic set of DNA repair-proficient and -deficient strains of B. subtilis, cured of all prophages, were constructed and analyzed for their sensitivities to selected mutagens. The results demonstrated that the lethal damage caused by ultraviolet (UV) radiation and by 4-nitroquinoline-1-oxide (4NQO) were repaired by the bacterial excision and/or recombination repair systems. In contrast, the lethal damages caused by ethyl methane sulfonate (EMS) and methyl methane sulfonate (MMS) were removed from the DNA by the recombination repair system of the bacteria, and not by the excision repair system. Significantly, the bacteria required both a functional recombination repair system and a functional excision repair system in order to remove the DNA damage caused by the bifunctional alkylating agent mitomycin C (MC).