Using the agar diffusion chamber (ADC) for colony formation of lymphocytic leukemic cells L7712, we compared the survival curves of the cells irradiated by three kinds of low LET radiations — 200kV x-rays,60,Co gamma rays and 20 MeV electron beam in two rates (0.35Gy/min and 1.0Gy/min).,The cells grew well in ADC both in the peritoneums of 615 inbred mice and Kumming white mice. The relationship between the number of cells implanted to each chamber and the number of colonies formed was shown to be linear. There was no significant difference between the two sets of data.,200kV x-rays irradiation to cells was studied. The survival curve for a dose rate of 0.35Gy/min had a ,D,0, of 1.55(1.45~1.67)Gy,n, of 0 94(0.88~1.03) and correlation coefficient ,r,= -0.88; while with a dose rate of 1.0Gy/min they were 1.39(1.30—1.54)Gy, 0.90(0.82~1.0) and -0.85 respectively. The ,D,0,n, and ,r, for survival curve after ,60,Co irradiation (0.35Gy/min) were 1.76(1.60 ~1.90)Gy, 0.96(0.87~1.07) and -0.82. The RBE of ,60,Co gamma rays as compared with 200kV x-rays was 0.88. The survival curve for 20MeV electron beam (1.0Gy/min) had a ,D,0, of 1.96(1.87~2.07)Gy and ,n, of 0.99 (0.94~1.05),r,=-0.93. Its RBE as compared with 200kV x-rays was 0 71.