Ataxia telangiectasia (AT) cells are defective in responding to damage induced by ionizing radiation. To study the modulation of double-strand break (DSB) repair by ionizing radiation and a defect in such modulation in AT cells, we compared processing of linearized shuttle vector pZ189 (linear DNA) by unirradiated or gamma-irradiated normal and AT lymphoblast hosts. The linear DNA processed in unirradiated AT and normal host cells yielded similar mutation frequencies in the supF-tRNA target gene. Irradiation of normal but not AT host cells decreased plasmid mutation frequency 2-fold if transfection occurred immediately. However, if transfection occurred 2 h after host cell irradiation, mutation frequencies increased 2-fold above those in unirradiated controls in both normal and AT hosts. DSB rejoining capability, based on the ratio of the number of progeny arising from equal amounts of linear DNA and supercoiled, undamaged pZ189, was 25- to 50-fold higher in normal than in AT hosts when both were unirradiated. Irradiation decreased DSB rejoining capability 2- to 5-fold in normal hosts but did not alter that of AT hosts. These findings demonstrate that AT cells normally rejoin DSBs as accurately as normal cells but do so less often, and that AT cells are defective in modulation of DSB rejoining by ionizing radiation.