Antioxidants mitigate radiation-induced lethality when started soon after radiation exposure, a delivery time that may not be practical due to difficulties in distribution and because the oral administration of such agents may require a delay beyond the prodromal stage of the radiation syndrome. We report the unexpected finding that antioxidant supplementation starting 24 h after total-body irradiation resulted in better survival than antioxidant supplementation started soon after the irradiation. The antioxidant dietary supplement was l-selenomethionine, sodium ascorbate, N-acetyl cysteine, α-lipoic acid, α-tocopherol succinate, and co-enzyme Q10. Total-body irradiation with 8 Gy in the absence of antioxidant supplementation was lethal by day 16. When antioxidant supplementation was started soon after irradiation, four of 14 mice survived. In contrast, 14 of 18 mice receiving antioxidant supplementation starting 24 h after irradiation were alive and well 30 days later. The numbers of spleen colonies and blood cells were higher in mice receiving antioxidant supplementation starting 24 h after irradiation than in mice receiving radiation alone. A diet supplemented with antioxidants administered starting 24 h after total-body irradiation improved bone marrow cell survival and mitigated lethality, with a radiation protection factor of approximately 1.18.