The monoclonal antibody RAP 5 immunoreactive with the ras gene product p21 was used in an immunohistochemical study of 57 patients with advanced ovarian cancer and in 28 normal ovaries. The pattern of the staining of various tumor specimens was similar to the germinal epithelium of normal ovaries, whereas the intensity of staining was more enhanced in carcinomas than in normal ovaries. However, we found a lack of correlation among staining intensity of RAP 5 and the histologic type, the histologic grade, the ploidy class, and the clinical outcome.