The radioprotective property of 50 mg/kg body weight jamun (Syzygium cumini) extract was studied in the cultured splenocytes of mice exposed to 0, 0.5, 1, 2, 3, or 4 Gy of γ-radiation. The spleens of irradiated mice were removed aseptically and the splenocytes were extracted from the individual spleens and cultured. The micronuclei were prepared 72 hours after irradiation in binucleate splenocytes by blocking cytokinesis with cytochalasin-B. Irradiation of mice resulted in a dose-dependent elevation in the micronucleated splenocytes. The exposure of mice not only elevated splenocytes bearing one micronucleus but also cells bearing 2 and multiple (>2) micronuclei indicating induction of complex DNA damage after irradiation. Oral treatment of mice with 50 mg/kg body weight of jamun leaf extract protected against the radiation-induced micronuclei formation. Jamun extract also protected against the formation of 2 and multiple micronuclei indicating repair or inhibition of complex DNA damage. The assessment of lipid peroxidation in mice brain homogenate has indicated a concentration dependent inhibition of lipid peroxidation by jamun extract. Studies in a cell free system revealed that jamun extract inhibited the formation of OH, O(2)-, DPPH, and ABTS(+) free radicals in a concentration dependent manner. Our study demonstrates that jamun extract protected mice against the radiation-induced DNA damage and inhibition of radiation-induced free radical formation may be one of the mechanisms of radioprotection.