The protective effects of Cysteine and MEA against X-irradiation were investigated, comparing the population of inoculated L-strain cells of the treated groups with that of the irradiated control groups on 2 days and 4 days after 800 r or 400 r irradiation. The results were as follows. 1) The protective effect of 10(-4)M Cysteine was observed significantly on 2 days after 800 r irradiation. 2) Cysteine of 10(-5)M was significantly effective on 2 days and 4 days after 800 r irradiation. 3) MEA seemed to be more effective than Cysteine (not significant).