The radioprotective effects of an acute administration of the isoflavone genistein (4¢, 5, 7-trihydroxyflavone), have been investigated in the present study. Male mice were administered with different doses (100, 200, 300, and 400 mg/kg body weight) of genistein 24 hours prior to 8 Gy gamma irradiation and the 200 mg/kg dose of genistein was determined to offer the maximum survivability and was used as an optimum dose for further experimentation. The 0.5 ml dose of genistein (200 mg/kg) was administered intraperitoneally to 2 different groups of mice, 15 minutes and 24 hours prior to gamma irradiation. In the mice treated with genistein with the optimum dose 24 hours before irradiation, a significant increase in 30 day survival has been recorded in contrast to the mice treated with genistein 15 minutes before the irradiation. The longer survivability (i. e. 20% for a period of more than 30 days) has been observed in the 24 hour group as compared to that of 15 minutes (i. e. 20% for 22 days). Although the radioprotective effect of genistein was evident in both groups, it was of greater magnitude in the group with a longer interval, indicating thereby an efficacy with longer retention with the possible minimum toxicity, unlike hitherto known other radioprotective agents.