The stomach region of hypocatalasemic mice of both sexes was X-irradiated once with a dose of 20 Gy. Thirteen months after the irradiation, 3 out of 13 (20%) males and 2 out of 9 (15%) females were observed to have developed signet ring cell carcinomas in the glandular stomach. This finding was statistically significant (P less than 0.01 in males and P less than 0.05 in females) compared to a total absence of similar tumors in the non-irradiated controls. Local invasion of malignant tumors into muscle and subserosal layers was observed, but no metastatic tumors were found in distant organs.