Water or aqueous ethanol extract of O. sanctum was given ip, either as a single dose or multiple doses, before a whole-body exposure to 11 Gy(LD100/30) of 60Co gamma radiation in albino mice. The water extract was more effective and less toxic than the aqueous ethanol extract. An optimum ip dose of 50 mg/kg (< 1/100 LD50) of the water extract, at 10 mg/kg/day for 5 consecutive days, gave the maximum survival. Increasing the dose per treatment or the number of treatments did not increase protection. Intraperitoneal administration gave the best protection (70% survival). Other routes (im, iv and po) were less effective and produced 37-47% survival. The optimum dose (ip) gave a dose modifying factor of 1.28. Since the extract may contain a number of chemical compounds, it is not possible to attribute the observed protection to any particular compound at present.