The mechanism that underlies doxorubicin-induced radiosensitization was investigated in a cell culture system. When V79 hamster cells were treated with doxorubicin, radiosensitization occurred as illustrated by an increase in α and a reduction in the mean inactivation dose (D) of the radiation survival curve. Under control conditions the radiation survival curve showed an α of 0·010 ± 0·02 and a D of 4·4 ± 0·2. After exposure to doxorubicin (1·5 μg/ml for 1 h prior to irradiation), α increased to 0·28 ± 0·04 and D decreased to 3·0 ± 0·3. Similar mean inactivation doses were found for doxorubicin doses of 0·75, 1·5 and 3·0 μg/ml, indicating that enhancement was not dose - dependent in the dose range studied. The influence of doxorubicin on radiation-induced DNA double-strand breaks (dsb) was then assessed under the same conditions as were used in the cell survival studies. Pretreatment with doxorubicin minimally affected radiation-induced dsb. However, doxorubicin increased dsb by 1·25 ± 0·07 under conditions in which repair was allowed to proceed for 45 min. These results show that doxorubicin inhibits the repair of radiation-induced dsb. Doxorubicin is known to change DNA conformation by the stabilization of DNA topoisomerase II complexes. It is possible that these changes alter the ability of repair enzymes to recognize and correct radiation-induced damage. © 1990 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.