The ability of WR-2721 to protect normal tissues against ionizing radiation is limited at its maximum protective dose owing to the toxicity of the drug per se. Previous studies had indicated that MPG (2-mercaptopropionylglycine) neutralizes to some extent the toxic effects of WR-2721 without impairing the protective efficiency. The effectiveness of two doses of WR-2721 (300 mg/kg and 150 mg/kg body weight) alone or in combination with an optimal dose (20 mg/kg body weight) of MPG, on the mouse bone marrow was studied by the exogenous spleen colony assay (CFU-s) after a single whole body exposure to 4.5 Gy of gamma radiation. Both the drugs individually increased the number of spleen colonies significantly above that of the irradiated control indicating higher stem cell survival. WR-2721 treatment gave better protection than MPG. MPG was more effective when administered within 5 min before or after irradiation than when given 30 to 25 min before irradiation. The combination of WR-2721, at either dose, with 20 mg/kg MPG gave an increase in the stem cell survival as compared to the single drug treatments and this effect was synergistic at 300 mg/kg WR-2721. MPG treatment within 5 min after irradiation produced a slightly higher CFU-s count than when the drug was injected before irradiation, though the difference was not statistically significant. It is concluded that in addition to the doses of the drug, the time of administration also could influence the effect of drug combinations.