The radiomimetic (chromosome-breaking) effect of streptonigrin (SN) in the roottips of Vicia faba is described and discussed in relation of the effects of other radiominetic agents. A 1-h treatment with 10 −5 M SN induced subchromatid exchanges in early prophase cells and chromatid aberrations in cells which were in middle or late interphase. The effect of SN was not inhibited by anoxia of 2,4-dinitrophenol and very little infleunce by temperature and pH during treatment.