The malignant potential of solid tumors is related to their ability to invade adjacent tissue and to metastasize. The plasminogen activation system is one of the critical factors in tumor progression since it is involved in tumor invasion and metastasis. This study was performed to examine the expression of u-PA in benign, borderline and malignant tumors of the ovary by immunohistochemical evaluation on formalin-fixed, paraffin-embedded specimens applying monoclonal antibody 3689 directed to the b-chain of u-PA. Normal epithelial cells of the ovary (n = 5) showed no staining of u-PA but some stromal cells were slightly stained. Invasive carcinomas (n = 16) and borderline tumors (n = 15) showed a moderate to strong diffuse cytoplasmic staining. Benign tumors (n = 20) showed a variety of staining. The observation of randomly positive u-PA stromal cells is noteworthy. The percentage of u-PA-positive tumors was higher in carcinomas than in other tumors. There was no correlation with other known risk factors of malignancy such as differentiation, stage or type of tumor. In conclusion there are noticeable differences in u-PA expression among ovarian tumors and u-PA increase in ovarian tumors can be attributed to an increased diffuse cytoplasmic content in the neoplastic epithelial cells.