Cathepsin B and L activities in cancerous and noncancerous mucosal tissues from 29 patients with gastric cancer were determined with a small amount of tissue homogenate. Both enzyme activities were significantly higher in cancerous tissues than in noncancerous tissues. The cathepsin B activity was higher with decreasing differentiation of the cancerous tissues, and also with increasing depth of invasion and metastasis to regional lymph nodes. Significantly high cathepsin B activity was observed in specimens of poorly differentiated adenocarcinomas, as well as in specimens from patients with extensive metastasis to n2 or n3 lymph nodes. These results suggest that high cathepsin B activity is characteristic of gastric cancer which invades and metastasizes. Therefore, in cases of marked elevation of cathepsin B activity in cancerous tissues, relatively extensive resection may be necessary to obtain a cure.