Mutation and killing caused by X-ray radiation and ,60,Co γ-ray radiation were studied in three different tryptophan-requiring auxotrophs (wp2, Wp2A, Cm 891) of ,Escherichia coli., These testers are sensitive to base pair substitution mutagens. Cm891 carries a R-factor and is more sensitive than wp 2 and wp2A to radiation-induced mutation and lethality. The results of our study show that (1)ionizing radiation was mutagenic to ,E. coli, (2) the order of mutagenic sensitivity among three strains to ionizing radiation was Cm 891,>,wp2A,>,wp2, (3) the dose rate of ,γ,-ray influence mutagenicity and lethalty of ,E. coli, strain, (4)the toxicity and mutagenicity of γ-ray were similar to X-ray when Cm891 was tested, however, γ-ray was more toxic and mutagenic than X-ray to wp2A ang wp2.