Potential effect of chemical radio-protectors, S-2-aminoethylisothiuronium bromide hydrobromide (AET) and 2-mercaptoethylamine (MEA), was tested by survival of ddY miceirrad iated with a lethal dose of X-rays, and characteristics of these compounds were comparatively examined. Both AET and MEA show comparatively acute toxicity and intraperitoneal injection of their lethal dose to mice results in their death within 24 hr. Comparison of the toxicity of AET and MEA, with consideration on their molecular weight, shows that the toxicity of AET is about twice stronger. Optimal period of the administration of AET and MEA for effective appearance of their protective power is 5-30 minutes before irradiation, and both were ineffective if given after irradiation. Toxicity and protective action of AET do not change if it is converted to mercaptoethylguanidine (MEG) by adjusting its moleous solution to pH 7.0 with sodium hydroxide. Protective effect of AET per aquecule is 2-4 times stronger than that of MEA. It was found that the protection of AET and MEA is not only effective against lethal action of X-ray irradiation but also markedly effective in protecting against weight loss.
