Mutagenicity of fasting gastric juice from 108 chronic gastritis patients in high- and low-risk areas of stomach cancer was assayed using in vitro SCE test in cultured human lymphocytes and micronucleus test in cultured Chinese hamster V79 cell. The results showed that mutagenicity could be found in this gastric juice. The positive rate of these two tests, SCE/cell, micronucleated cell rate and micronucleus rate were significantly higher in high-risk area than in low-risk area (P less than 0.01). There was a tendency that the mutagenic activity was higher in high pH than in low pH gastric juice and in males than in females. In high-risk area, the mutagenicity of gastric juice was obviously higher in younger than in elder aged group. Because no S9 was added in this test system, the results seem to suggest that the mutagenicity found in the gastric juice of patients with chronic gastritis might be, at least in part, related to direct-acting N-nitroso-compounds synthesized in the stomach.