The ,60,Co γ-ray induced single strand breaks of DNA and their rejoining in cultured chinese hamster lung (CHL) cells and mouse spleen cells isolated freshly from mice were detected by hydroxylapatite chromatography. The method is based on the observation that the rate of denaturation of DNA to single strands in alkali is increased after irradiation, the interpretation being that this effect is due to DNA strand breaks. The percentage of labeled DNA that remained double strand after 30 min alkali treatment was inversely proportional to the radiation dose within the range of 1-10Gy for the two kinds of cells.,The DNA strand breaks in unirradiated spleen cells was more than in unirradiated CHL cells, but the yield of breaks induced per 0.01Gy was found to be similar in the spleen cells and CHL cells. In contrast with these findings, most DNA strand breaks in the CHL cells were rejoined within 30min, the breaks were rejoined faster in the cells incubated in medium with 20% new born calf serum after irradiation than in medium without any serum. The repair of damaged DNA could not be detected in the spleen cells cultured in RPMI 1640 medium without any serum.