The radioprotective drug, WR-2721 (S,2-[3-aminopropylamino]ethyl-phosphorothioic acid), has been studied in terms of its ability to (a) protect mice against mechlorethamine (HN2)-induced hematopoietic death, and (b) alter the ability of HN2 injections to induce growth delay in a solid tumor, the Line 1 lung carcinoma. When WR-2721 was injected ip 15 minutes before iv injections of HN2, it increased resistance to hematopoietic death by a factor of 2, and the protection declined with a half-life of 1.5-2.0 hours. Similar administration of both drugs failed to alter the responsiveness of the Line 1 lung carcinoma to HN2-induced growth delays, except when the HN2 was given within 15 minutes after WR-2721. This interaction of the two drugs, when given within 5-15 minutes of each other, does not appear to be true protection at the tumor site, but rather appears to result from HN2 inactivation in the blood. When HN2 is given 30-60 minutes after WR-2721, it is possible to obtain a twofold increase in the tumor delay without risking increased hematopoietic injury.