Although WR-2721, S-2-(3-aminopropylamino)ethylphosphorothioc acid, is the most widely studied and most effective radioprotective drug at present, it is nevertheless clear from animal studies that it has important shortcomings as the ideal radioprotector in clinical radiotherapy. More effective and less toxic radioprotective drugs are needed. For this reason, a chemical radioprotector screening program has been initiated at the Fox Chase Cancer Center under a contract with the National Cancer Institute. Most of the 20 compounds that have now entered the screening program provide good protection of the mouse hematopoietic system as indicated by 30 day survival following the radiation LD100/30. Administration of a radioprotector dose equal to one half of the maximum tolerated dose (MTD/2) gave hematopoietic dose reduction factors (DRF's) as high as 2.3. No radioprotector appeared to be superior to WR-2721, although four others gave DRF's exceeding 1.8.