Cathepsin B and L activities in cancer tissue and non-cancerous mucosal tissue were determined with small amounts of tissue homogenate from 29 operated patients with gastric cancer. Both enzyme activities were significantly higher in cancer tissues than in noncancerous mucosal tissues. The level of cathepsin B activity was higher in specimens of poorly differentiated adenocarcinomas, deeply invaded ps (+) and extensively metastasized regional lymph nodes (n(2) or n(3)) than in specimens of tubular adenocarcinomas, invaded ps (-) and no metastasized regional lymph nodes. No significant correlation was observed between the elevated levels of cathepsin L activity in cancer tissues and any of the histological findings. These results suggest a close relationship of this enzyme to local invasion and metastasis to regional lymph nodes.