The relationship between deoxyribonucleic acid (DNA) damage and the cell death 
induced by gamma-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.