Malignant tumors derived from 60 patients with primary ovarian cancer were assayed for lysosomal cysteine proteinases--cathepsins B and L (Cat B, Cat L) and lysosomal aspartyl proteinase--cathepsin D (Cat D) as compared with benign ovarian tumors in controls. Malignant ovarian tumors exhibited significantly increased Cat B and Cat L activity (18.5- and 9-fold, respectively, p < 0.001) and moderately increased Cat D activity (1.5-fold, p < 0.01) as compared with those of benign tumors. The data obtained indicate that malignant ovarian tumors with a high Cat B activity (higher than the critical level of 723.5 +/- 100.0 Opmol/min. mg protein) proved to be more malignant in nature (low cell differentiated tumors predominated with early metastases spreading and relapses). Retrospective analysis of the disease development in 38 patients with ovarian tumors revealed that patients with early relapses (within the first year after surgery) demonstrated higher Cat B activity in tumors as compared with both the critical level and with that of patients without relapses; in the latter group the increase was found to be 2.4-fold (p < 0.01). The rate of CatB activity increases proved to be 56% and 8% in patients with and without relapses, respectively. Therefore, evaluation of the critical level of CatB activity in patients with primary ovarian cancer may be employed for prognostic purposes.