Mutagenicity of the gastric juice in patients with chronic gastritis was determined by reverse mutation test using Salmonella typhimurium TA100 and investigated the relationship between intestinal metaplasia which has been regarded as a precancerous lesion (candidate) of gastric cancer, and atrophy of gastric mucosa. The degree of intestinal metaplasia was quantified in six biopsy specimen for one patient and represented as metaplasia index (MI). The Ames ratio, mutagenicity, of gastric juice showed no significant relationship with the age of patient. On the other hand, Ames ratio and MI, and the age of patient showed significant relationship, respectively (p < 0.01). Furthermore, patients with more intensive atrophy tended to show higher Ames ratio. As shown above, mutagenicity of the gastric juice suggested some relationships to the development of intestinal metaplasia and atrophy of gastric mucosa.