This paper deals with the radioprotection afforded by selenocystine, selenomethionine, colloidal selenium, selenoxanthene, selenoxanthone, and selenochromone in living rats. Our evaluation of radioprotection was based on the ability of the compounds to alleviate the mortality and the leukopenia occurring in rats exposed to irradiation of 600, 750, and 900 R. The results show that these compounds have an action similar to, and sometimes even superior to, that of cysteine. A preliminary study was made of the mechanism of action of the selenium derivatives by measurement of the retention of selenium in the blood, and its distribution at the subcellular level in the liver.