This paper reports on the ability of the radioprotector WR-2721 to protect bone marrow colony forming units from the cell kill of various cytotoxic chemotherapy agents including nitrogen mustard, cyclophosphamide, BCNU, cis-platinum, and 5-fluorouracil. The dose-modifying factors seen with these compounds ranged from 1.5 to 4.6. Protection by WR-2721 seems to be selective for the bone marrow endpoint, in that similar protection against EMT6/SF tumor cell kill was not seen with the same combination of the cytotoxic chemotherapy agents and WR-2721 as measured by regrowth delay. The implications of these data are discussed in the paper.