Radioprotection by 2,3-dimethyl-6-(2-(dimethylamino)ethyl)-6H-indolo[2,3-b]quinoxaline (B220) on survival and growth of female C3H mice exposed to acute whole-body gamma-radiation was evaluated for 7.5-8 months following irradiation in two separate experiments. For adult (12 weeks old) mice, B220 administration increased median survival after 8 Gy by a factor of 1.27 when given within 24 h pre-irradiation, administration up to 24 h post-irradiation had a similar effect (1.20) but in addition resulted in 1 of 9 (11 %) mice alive after 32 weeks. For adult mice irradiated with 10-14 Gy, B220 had no significant effect on survival. For very young mice (4 weeks old), however, B220 administration within 24 h pre-irradiation protected from growth retardation at both 1 and 6 Gy, and from gray-hairing at 6 Gy. In conclusion, the well tolerated drug B220 offered radioprotection in both studies and its administration could be further optimized.