Cysteine greatly reduces the sensitivity of rats and mice to lethal amts. of X-rays delivered at either high or low dose rates provided that the amino acid is given before the exposure. Rather comparable protection is obtained when cysteine is injd. intraven. in rats immediately or 1 hr. before irradiation with 800 r (90% reduction in the 28-day mortality). Injn. immediately after exposure or 6 or 24 hrs. before is without influence. Cysteine also improves survival significantly when it is given orally 30-60 min. prior to the irradiation. Glutathione (intraven. but not oral) can also diminish radiation toxicity. Cysttne, methi-onine, ascorbic acid, and Na2S do not alter survival of the irradiated animal. Results with colloidal S are equivocal. The possible significance of these observations is discussed.