Chapman, Sipe, Eltzholtz, Cronkite, and Chambers (1) first demonstrated the beneficial effect of glutathione injection on the x-ray induced mortality rate and weight loss in mice. We found the same effect in guinea-pigs of 200 to 300 gm. body weight on exposure to 200 to 250 r. Each animal was injected with 100 mg. glutathione immediately before and after irradiation. Glutathione concentration ranges in normal guinea-pigs from 100 to 140 mg. per 100 ml. erythrocytes, with only approximately 8 to 10 per cent loss of stability, following incubation with acetylphenyl hydrazine. In irradiated nonprotected controls, the glutathione concentration reaches extremely low levels, and the glutathione loses its stability (approximately 50 per cent and more), while there is a considerable depression of the leukocytes in the peripheral blood. In the irradiated and protected animals the glutathione erythrocyte levels were found to be considerably higher, though there is likewise some loss of stability. Glutathione stability depends on reduced tri-phosphopyridine nucleotide (TPN); therefore, glucose 6-phosphate dehydrogenase and 6-phospho-gluconic dehydrogenase activity were also studied in preliminary experiments to investigate the loss of stability. This is measured by the difference in optical density per gram of hemoglobin per minute. In the nonprotected group it was lower than in the protected; the lowest value in the former was 4.02, the highest in the latter 11.9. Most striking results were recently obtained in a small series of experiments with protection due to injection of the sodium salt of ribonucleic acid. Nine guinea-pigs were irradiated with 250 r (LD 50 dose 200 r) and injected with 100 mg. sodium ribonucleate before and after irradiation; 9 animals without protection by ribonucleic acid served as controls. Seven of the nonprotected animals died within ten to twenty-seven days following irradiation, while only 3 of the protected animals died ten days after irradiation, one of them, however, on the third day. This fatality cannot be attributed to radiation damage. The survivors were still alive forty days after irradiation and in perfect condition. The 2 survivors were re-irradiated with 275r, and 6 of them were again protected with 100 mg. sodium ribonucleate before and after irradiation. The 2 nonprotected guinea-pigs died twelve days after irradiation; 4 of the protected animals were still in perfect condition after four weeks. At the present time experiments are in progress, using a low-molecular-weight ribonucleic acid, prepared from yeast according to Monier et al. (2). This will be studied in normal guinea-pigs as well as in tumor-bearing mice. It is possible that the depression of the glutathione level in the erythrocytes might be accompanied by simultaneous increase of sulfhydryl-containing precursors, such as l-cysteine or l-glutamylcysteine.