[en] In the course of the investigations of the biological effects of radionuclides incorporated in DNA single (SSB) and double strand breaks (DSB) caused tritium-decay were measured and compared with respective data resulting from ¹²⁵I. Tritium bound to thymidine and iododeoxyuridine seems to be more effective than tritium bound to other DNA-precursors. On the basis of decay, methyl-³H thymidine appears to be more effective with regard to the production of strand breaks than ³H in position 6 of the pyrimidine ring. Based on the numbers of strand-breaks per rad, position 6 is more effective in accordance with data obtained by F. Krasin et al. The ratio of SSBs to DSBs per tritium decay appears to be approximately 8 in mammlian cells. Not only SSBs but also DSBs induced by ³H in mammalian cells are reapairable. (orig./AJ)