The efficacy of the radioprotective effect of Callophyllis japonica ethyl acetate (CJEA) extract wasstudied by comparing it to that of amifostine, a well-known radioprotective agent, and byevaluating the dose reduction factor, an indicator of radioprotective efficacy. Pretreatment withCJEA extract (100 mg/kg body weight) prior to receiving 12 Gy irradiation significantly improved thesurvival of jejunal crypts at 3.5 day post-irradiation, but attenuated the level of malondialdehydecompared to vehicle alone (P < 0.01). A similar gastroprotective effect was also obtained in theamifostine-treated irradiated group (P < 0.01). The efficacy of the radioprotective effect wasfurther confirmed by the dose reduction factor, 1.41. Collectively, these results suggest that CJEAextract is a useful radioprotectant whose efficacy is similar to that of amifostine and whoseradioprotective mechanism is in part the reduction of lipid peroxidation caused by gammairradiation.