Effects of gamma-irradiation, given in the range of 5 to 30 Gy on Caski cells (Epitheloid carcinoma from the cervix) were investigated by the MTT (3-(4,5-dimethylthiazol-2-yl) -2, 5-diphenyl tetrazolium bromide) method. Results were compared with data assessed simultaneously from cell number counts. The sizes of cells irradiated with 10 to 30 Gy were larger than those of unirradiated ones, and each irradiated cell reduced a larger amount of MTT than did each unirradiated cell. Irradiation in the above range, therefore causes Caski cells to lose their ability to divide, but the effect on the mitochondria was very slight. Application of the MTT method to the irradiated cells should be done with care. Because, in the irradiated cells depending on the irradiation dose, the MTT activity does not correlate to the cell number.