Radioprotective activity of pure compounds isolated from the plant Phyllanthus amarus was studied using rat liver mitochondria and pBR322 plasmid DNA as an in vitro model system. These compounds were ellagitannins namely amariin, 1-galloyl-2,3-dehydrohexahydroxydiphenyl (DHHDP)-glucose, repandusinic acid, geraniin, corilagin, phyllanthusiin D, and flavonoids namely rutin, and quercetin 3-O-glucoside. The activity was then correlated with their hydroxyl and superoxide radical scavenging activity. Both ellagitannins and flavonoids effectively prevented lipid peroxidation and protein oxidation in mitochondria. The compounds also prevented radiation induced single strand breaks in pBR322 plasmid DNA. The radioprotective activity of ellagitannins and flavonoids could be due to their ability to scavenge different radicals more or less efficiently, relieving the oxidative stress. Protection conferred by flavonoids, rutin and quercetin 3-O-glucoside to rat liver mitochondria and plasmid pBR322 DNA from radiation induced damage was due to their strong hydroxyl radical scavenging activity. The inhibitory effect of ellagitannins on lipid peroxidation in liver mitochondria was due to their efficient superoxide radical scavenging ability. This is the first report about the radioprotective activity of pure ellagitannins from Phyllanthus amarus.