Oral administration of an algal mutant C. vulgaris E-25, 1 hr before or immediately after exposure to sublethal gamma-rays increased the number of endogenous spleen colony forming units (E-CFU). The magnitude of radioprotection was dependent on both, the dose of C. vulgaris fed and the time of administration. An optimal E-CFU was observed when 500 mg/kg body wt. of C. vulgaris was fed 1 hr before or immediately after irradiation. Significant recovery was observed in the number of bone marrow cells and the spleen weight. LD50/30 for Chlorella pre- and post-treated mice were 8.66 and 9.0 Gy, respectively compared to the control value of 7.8 Gy. The dose reduction factor (DRF) was 1.11 and 1.15 for pre-treated and post-treated mice respectively.