S-2-3-aminopropylamino ethyl phosphorothioic acid hydrate was studied in rodents and primates to determine its radioprotective capacity. Thirty minutes before being irridated with 1,200 R cobalt-60, male Sprague- Dawley rats were injected intraperitioneally with sterile water solutions containing 250 mg.kg. body weight of the radioprotective agent. In at least one group of 12 rats, survival at 30 days postirradiation was 100. All similarly irradiated control rats without medication died. This study was extended to standardized primates, Macaca mulatta. The primates were injected intravenously with the drug in sterile saline solution 30 minutes before exposure to 850 R x- rays. Drug concentrations ranged from 250 to 400 mg.kg. At the two best concentrations 250 and 300 mg.kg., 12 out of 15 treated primates survived 30 days. All 8 similarly irradiated control primates died between the tenth and fifteen day postirradiation. Pathology findings are presented for each group.