The effect of various doses (0, 25, 50, 75, 100, 125, 150, 300, 600 and 900 mg kg-1) of the alcoholic extract of the plant Ageratum conyzoides Linn. (ACE), on the alteration of radiation-induced mortality in mice exposed to 10 Gy of gamma radiation was studied. The acute toxicity studies showed that the drug was non-toxic up to a dose of 3000 mg kg-1, the highest dose that could be tested for acute toxicity. Administration of ACE resulted in a dose-dependent decline in radiation-induced mortality up to a dose of 75 mg kg-1, the dose at which the highest number of survivors (70.83%) was observed. Thereafter, the number of survivors declined with increasing doses of ACE and a nadir was reached at 900 mg kg-1 ACE. Since the number of survivors was highest for 75 mg kg-1 ACE, this was considered the optimum dose for radioprotection and used in further studies in which mice were treated with 75 mg kg-1 ACE before exposure to 6, 7, 8, 9, 10 and 11 Gy of gamma radiation. The treatment of mice with 75 mg kg-1 ACE reduced the severity of symptoms of radiation sickness and mortality at all exposure doses, and a significant increase in survival was observed compared with the non-treated irradiated group. The ACE treatment effectively protected mice against the gastrointestinal as well as bone marrow related death, as revealed by the increased number of survivors at all irradiation doses. The dose reduction factor was found to be 1.3. To understand the mechanism of action, various doses of ACE were evaluated for their in-vitro scavenging action on 1,1-diphenyl-2-picrylhydrazyl (DPPH), a chemically stable free radical. ACE was found to scavenge DPPH radicals in a concentration-dependent manner, indicating that the radioprotection afforded by ACE may be in part due to the scavenging of reactive oxygen species induced by ionizing radiation.