ABSTRACT. The relationship between deoxyribonucleic acid (DNA) damage and the cell death induced by γ-irradiation was examined in three kinds of cells, Chinese hamster ovary fibroblast CHO-K1, human melanoma HMV-II and mouse leukemia L5178Y. Cell survival was determined by a clonogenic assay. The induction and rejoining of DNA strand breaks induced by radiation were measured by the alkaline and neutral comet assays. L5178Y cells were the most radiosensitive, while CHO-K1 cells and HMV-II cells were radioresistant. There was an inverse relationship between the survival fraction at 2 Gy (SF2) and the yield of initial DNA strand breaks per unit dose under the alkaline condition for the comet assay, and also a relationship between SF2 and the residual DNA strand breaks (for 4 hr after irradiation) under the neutral condition for the comet assay, the latter being generally considered to be relative to cellular radiosensitivity. In the present analysis, it was considered that the alkaline condition for the comet assay was optimal for evaluating the initial DNA strand breaks, while the neutral condition was optimal for evaluating the residual DNA strand breaks. Since the comet assay is simpler and more rapid than other methods for detecting radiation-induced DNA damage, this assay appears to be a useful predictive assay for evaluating cellular clonogenic radiosensitivity of tumor cells. KEY WORDS: comet assay, DNA strand break, γ-irradiation, predictive assay, radiosensitivity.J. Vet. Med. Sci. 65(4): 471-477, 2003 Different types of cell lines show different radiosensitivities. In addition, even a tumor of the same origin and pathology, can show radiosensitivity over quite a wide range [4], and therefore, understanding the radiosensitivity of an individual tumor is of great importance in the clinical situation. Knowledge of the intrinsic radiosensitivities of cells from a tumor biopsy can help produce an optimized radiotherapy protocol for individual patients.Reproductive death measured by a clonogenic assay is an important index for the cell death induced by radiation, and the cellular radiosensitivities of fibroblasts and tumor cells have been evaluated by a clonogenic assay [13,17]. A good correlation was found between the reproductive death of fibroblasts measured by the clonogenic assay and the severity of late normal tissue effects, such as fibrosis and erythema, after radiotherapy [7,14]. However, this clonogenic assay consumes a lot of time, and the cells must be proliferated in vitro [4,14]. Hence this assay is not optimal for routine clinical use, and thus there is now much interest in exploiting a more rapid and simple assay to evaluate cellular radiosensitivity [8,25].It is said that the cell death caused by radiation is due to deoxyribonucleic acid (DNA) damage [29]. DNA double strand breaks (dsbs) are considered to be closely related to cell death because it is suggested that they can lead directly to chromosome aberration and the loss of genetic material [22]. To date, the correlation between radiose...