Activity of lysosomal proteinases cathepsin B, L and D was studied in tissues of malignant tumors, cancer and normal lymph nodes and mucosal membrane obtained from 18 patients with gastric cancer. The enzymatic activity was distinctly higher in cancer tissues as compared with controls. Activity of cathepsin D was increased in tissues with diminished rate of the tumor differentiation, with pronounced cancer invasion and metastases to regional lymph nodes--the group of highly negative prognosis. At the same time, activity of cathepsin B was increased in tissues of patients with positive prognosis of gastric cancer; negative correlation of cathepsin B activity was observed in both groups of patients. This correlation may be considered as an additional prognostic factor.