cysteamine unequivocally protected against mortality, the magnitude of the protective action depending on the accumulated exposure and on the time interval between fractions. At low accumulated exposures radiation injury was insufficient to show significant differences between protected and control animals, while for high exposures radiation injury was supralethal; also, the effect of fractionation intervals was often variable. No single dose-reduction factor could be de- rived from these experiments since the values of this factor vary in each series with exposure, fraction size and fractionation interval. However, cysteamine was shown to protect not only against the acute injuries but also against fractionated doses in the sublethal range. Any more precise assessment would be unwarranted owing to the toxicity of the drug and to the adverse effect of the administration procedure which influenced the survival of the animals rather substantially. 306. Yuhas [Y1] reported that the radioprotective agent WR-2721 [S - 2-(3- amino propylamino) ethylphosphorothioic acid] protects against acute death more efficiently than it can protect against the life-shortening effects of radiation, although the exact extent of this protection could not be directly and precisely estimated. It has, however, been shown [D6] that the ability of the drug to pro- tect against life-shortening varies with the size of the dose of radiation. 307. Storer [S30] investigated on A/J and C57BL/6J male and female mice the effect of four radioprotectors administered i.p. 15' prior to irradiation. They were: paraaminopropiophenone (PAPP) at 40 mg/kg, mercapto-ethylamine (MEA) at 200 mg/kg, amynoethylthyouronium (AET) at 200 mg/kg and 5-hydroxytryptamine (5 - HT) at 100 mg/kg. X rays of 300 kVp were given acutely at 150, 300, 600 R to the control animals and at proportionately higher exposures to the protected mice. Dose-reduction factors in the region of 1.5 - 1.8 were found for the various drugs with respect to the LD of the x rays and the drug treatments had no significant effect on the lonevity of the non-irradiated mice. Within the range of exposures tested, mean survival time decreased as a function of dose (with some sex and strain differences in the amount of response) with concave upward relationships, although the hypothesis of linearity could not en- tirely be rejected. The pooled data (all strains and sexes and drugs together) for control and for protected mice showed that mean life-shortening was a curvilinear function of dose both with and without drugs and that the radioprotective treatment did afford some protection against life-shortening. extent of protection varied with strain, sex and drug. However, the PAPP was found to be the