Gastrointestinal damage associated with radiation therapy is currently an inevitable outcome. The protective effect of Kefir was assessed for its usefulness against radiation-induced gastrointestinal damage. A Kefir supernatant was diluted by 2- or 10-fold and administered for 1 week prior to 8 Gray (Gy) X-ray irradiation at a dose rate of 2 Gy/min, with an additional 15 d of administration post-irradiation. The survival rate of control mice with normal drinking water dropped to 70% on days 4 through 9 post-irradiation. On the other hand, 100% of mice in the 10- and 2-fold-diluted Kefir groups survived up to day 9 post-irradiation (p<0.05 and p<0.01, respectively). Examinations for crypt regeneration against 8, 10 and 12 Gy irradiation at a dose rate of 4 Gy/min revealed that the crypt number was significantly increased in the mice administered both diluted Kefir solutions (p<0.01 for each). Histological and immunohistochemical examinations revealed that the diluted Kefir solutions protected the crypts from radiation, and promoted crypt regeneration. In addition, lyophilized Kefir powder was found to significantly recover the testis weights (p<0.05), but had no effects on the body and spleen weights, after 8 Gy irradiation. These findings suggest that Kefir could be a promising candidate as a radiation-protective agent.