Saccharopolyspora erythraea, 963 with antibacterial activity was a mutant strain which obtained by ,60,Co γ-rays irradiation on ,Saccharopolyspora erythraea, M. In this experiment,Saccharopolyspora erythraea, 963 was also radiated by ,60,Co γ-rays to study repeated mutagenic effects. The results indicate that the lethal effect of strain 963 is lower than that of starting strain M in the same absorbed doses of irradiation. After the repeated mutagenesis, the rate of morphological variation of strains was up to 24.1‰ and equal to 4.8 times as that of starting strain M. The inhibitory effect of positively mutated strain increased by 40% and the rate of positive mutation was 2.01‰ and rose by 20.36% compared with that of the strain 963. The method of repeated mutagenesis is helpful in improving screening efficiency of mutation breeding as the morphological variation of mutated strain and positive mutation can be obtained.