[en] Accumulation of DNA strand breaks has been studied in mammalian cells after incorporation of ³H-thymidine (³H-TdR), ¹⁴C-thymidine (¹⁴C-TdR) and ¹²⁵Iododeoxyuridine (¹²⁵IUdR). In the experiments the cells were incubated at 37 degrees C followed by DNA strand break analysis by the DNA unwinding method. ¹²⁵ IUdR was extremely efficient to induce unrepairable DNA strand break while ³H-TdR and ¹⁴C-TdR showed a much smaller effect. The relative Biological Effectiveness (RBE) for ¹²⁵I in relation to ³H was found to be 7-13. The frequency of chromatid-type aberrations in Chinese hamster cells was investigated after incorporation of ²H-TdR and ¹²⁵IUdR. RBE for ¹²⁵I in relation to H wascalculated at low doses for (i) chromatid aberrations (17+-6), (ii) the sum of isochromatid aberrations and chromatid exchanges (21+-9) and (iii) the total number of chromatid aberrations (18+-5). Our results are consistent with the assumption that chromosomal aberrations mainly originate from unrepaired DNA strand breaks. ¹²⁵I-triiodothyronine (¹²⁵I-T3) binds to a high-affinity, low-capacity nuclear receptor close to DNA and thereby induces serious DNA damage in GH1 abd Chinese hamster cells. The efficiency of inducing DNA strand breaks was as high as after incorporation of ¹²⁵IUdR implying and extremely high radiotoxicity for ¹²⁵I-T3. A method based on accumulation of DNA strand breaks was established to estimate radiotoxic effects ¹¹C-labelled compounds. As a model system ¹¹C-methionine was chosen. The number of unrepaired DNA strand breaks was examined by the DNA unwinding method after incubation of Chinese hamster cells at 37 degrees C for 60 minutes in ¹¹C methionine containing medium. RBE of ¹¹C-methionine was estimated to be slightly above 1. With 60 refs. (author)