The shape of the survival curve for cells inactivated by tritium decay in DNA is modified by the presence of halogenated pyrimidines in the DNA in a manner analogous to their effect on X-ray induced reproductive death. The large shoulder found for tritium suicide is removed completely by coincorporation of 10(-6) M IUdR. The oxygen enhancement ratio for 125I and tritium disintegrations in unsynchronized Chinese hamster cells was determined for cells permitted to accumulate damage from these events at 4 degrees C. The oxygen enhancement ratio for 125I induced damage is 1.4. This is much smaller than the OER found for tritium decay which is similar or more than that found for X-ray exposure under the same conditions. These results suggest that the nature of the lesions produced by 125I decay in DNA are analogous to those produced by high LET radiation while those lesions produced by tritium are similar to lesions produced by roentgen rays. In synchronous V79 cells the effects of 125I induced damage in different regions of the mammalian cell DNA was examined taking advantage of the fact that DNA replication in hamster nuclei follows a time-dependent three dimensional pattern. The experiments indicate that 125I decays accumulated in the G2-period of the cell cycle have different efficiences for the induction of reproductive death depending on the region of the DNA which is labeled. The efficiency for the induction of reproductive death appears to be a maximum in DNA that replicates in V79 cells near the end of the DNA replication cycle. Electron capture events are dramatically efficient in the production of lethal chromosome aberrations. In CHO cells synchronized in the G1-stage of the cell cycle stored in the frozen state the efficiency for the induction of dicentric and ring chromosomes is 0.03. The dose response curve for the induction of these aberrations is linear in contrast to the curvilinear response found for roentgen ray exposure under the same conditions. Data on this kind suggest that there may exist "critical" regions within mammalian cell nuclei where chromatin fibers from two different chromosomes are in close proximity to each other and both are damaged non-repairably by a single electron capture event.