Background: Although numerous natural or synthe c drugs have been tested for their radioprotec ve capacity, yet no suitable drug has been introduced for rou ne clinical use. In this study the radioprotec ve effect of "a new herbal immunomodulator" commercially known as IMOD, specifically made to decrease the side effects of HIV virus, was inves gated on mouse bone marrow cells. Materials and Methods: Female NMRI mice (in a group of five) were exposed to 2 Gy gamma radia on following three days of intravenously injec on (IV) of IMOD at various doses. Mice were sacrificed 48 and 72 h a1er irradia on. Bone marrow was flushed and slides for bone marrow smears were prepared according to standard method. A1er staining slides in May Grunwald and Giemsa, polychroma c erythrocytes (PCE) and normochroma c erythrocytes (NCE) were scored for presence of micronucleus (MN). Results: The results showed that gamma irradia on increased the frequency of micronuclei drama cally and excreted cytotoxic effect of cell prolifera on. Injec on of various doses of IMOD before irradia on however, led to a considerable reduc on in the frequency of micronuclei in bone marrow erythrocytes as well as cellular toxicity. Conclusion: Results indicated radioprotec ve capability of IMOD with a dose reduc on factor (DRF) of about 2.3 at a dose of 20 mg/kg body weight. The considerably high DRF might be indica ve that IMOD besides being an immunomodulator might also posses' an oxidant property.Keywords: R adioprotection, im m unom odulator, I M OD, m ouse bone marrow, micronucleus assay.