Chinese hamster DDN cells have been exposed to low doses of X-irradiation (< 15 Gy). These doses have been shown to induce significant levels of repair replication. Levels of activity of DNA polymerase were subsequently measured in cells exposed in G1 phase of mitosis to 7.5 Gy of X-rays. Following exposure, the activity of this enzyme was observed to rise to a peak within 15 min post-irradiation, then decline and then rise again over a long period. The initial rise at least was found to be sensitive to inhibition by N-ethylmaleimide, a known inhibitor of DNA polymerase-alpha and DNA polymerase-gamma at the concentration used. It is therefore suggested that repair of X-ray-induced damage in these DON cells was at least in part dependent upon induced DNA polymerase activity and this may involve DNA polymerase-alpha or DNA polymerase-gamma, or both.