The possible role of orally fed Chlorella vulgaris (E-25) in modulating the gamma-ray induced chromosomal damage in whole-body irradiated mice was evaluated using a micronucleus test. Different doses of E-25 were administered either chronically (once, twice or thrice a day for 28 days) or as single acute doses before/after irradiation. A significant radioprotective effect was observed in both acute and chronic pretreatments, but only at doses above 400 mg/kg body weight. However, in mice that received E-25 (500 mg/kg) three times a day for 28 days, there was no protective effect, and a significant loss in their body weight was observed. Interestingly, E-25 afforded significant radioprotection even when it was administered within 0.4 hr after irradiation.