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 G 1 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 postirradiation, 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-o< and DNA polymerase-y 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-o< or DNA polymerase-y, or both.