In the last years the attractive hypothesis of a possible involvement of mammalian topoisomerases in DNA repair has been proposed, given their molecular mechanism of action. So far, using asynchronous cultures a lot of controversial results have been reported, without taking into account the frequently dramatic fluctuations of topoisomerase activities depending upon the cell cycle stage and proliferation rate (mainly for topoisomerase II). We have addressed this question making use of G1 synchronous cultures of the Chinese hamster radiosensitive mutants xrs 5 (defective in DNA double strand breaks rejoining) and irs 2 (which shows radioresistant DNA synthesis), as well as their parental lines CHO K1 and V79 respectively, which show a normal radiosensitivity. Cells were irradiated with 5Gy of X‐rays and the activities of topoisomerases I and II in nuclear extracts were studied for comparison with non‐irradiated controls in both the mutants and parental cell lines. Our results clearly show a modulation of the topoisomerase activities after irradiation, that varies depending upon the mutation that the different lines bear. While this hypothesis needs further testing, an interesting idea is that DNA topoisomerases might be involved in the cellular response to radiation damage, either through a direct participation in the repair mechanisms or in a preparative step to allow repair to proceed.