Activity of lysosomal proteinases cathepsins B and D was studied in normal tissues, malignant and benign ovary tumors, in blood sera and ascites fluid of patients with primary ovary cancer. The cathepsin B activity was 12-fold higher in tissues of ovary malignant tumors as compared with that of normal ovary and 6-fold higher -- as compared with tissues of benign tumors. Activation of the enzyme depended directly on the mass of malignant tumors and on the intensity of their penetration into surrounding tissues (uterus, urinary bladder, small pelvic fat) and spreading over peritoneum. Activity of cathepsin D was 2- and 2.6-fold higher in malignant ovary tumors as compared with normal tissue and benign tumors, respectively. The data obtained suggest that estimation of cathepsins B and D activities in bioptic material from ovary tumors might be useful for diagnosis of the disease.