The protective effects of AET, Cystamine and HCT against irradiation were studied, comparing the cell population of the treated groups with that of the irradiated control groups on 2 days and 4 days after 400 r or 800 r irradiation. The results were as follows. 1) The protective effect of 10(-4)M AET was observed significantly in both 800 r and 400 r irradiated groups. 2) The protective effect of 10(-3)M AET was observed on 2 days and 4 days after irradiation. 3) The protective effects were not observed in Cystamine and HCT treated groups.