Two flavonoids, orientin and vicenin, isolated from the leaves of the Indian plant Ocimum sanctum were tested for their radioprotective effect in mice. Both compounds provided protection against death from gastrointestinal syndrome as well as bone marrow syndrome when injected intraperitoneally (i.p.) before whole-body exposure to 11 Gy gamma radiation. The optimum drug dose for protection was 50 microg/kg body weight: An increase in the drug dose did not increase protection. No acute toxicity was observed at doses as high as 100 mg/kg body weight of either compound. Maximum protection was obtained when either compound was injected i.p. 30 min before irradiation. Changing the route of administration or the interval between drug injection (i.p.) and irradiation reduced protection. Drug treatment after irradiation was not very effective. Vicenin was slightly better than orientin in increasing survival at 30 days; protection by vicenin also lasted longer. Dose modification factors (DMFs) for the LD50 were 1.37 for vicenin and 1.30 for orientin. Radical scavenging activity has been demonstrated for both orientin and vicenin, and this appears to be one of the mechanisms of protection by these flavonoids.