Expression of ras p21 oncoproteins in human ovarian carcinomas was examined immunohistochemically by using a monoclonal antibody(clone RAS 10) with respect to the degree of their histological differentiation. To achieve this, the intensity of staining for the protein was compared between undifferentiated and well differentiated carcinomas, i.e. extreme subtypes of common epithelial carcinomas. The former was composed of 8 "solid" carcinomas and the latter, 11 serous, 8 mucinous, 4 endometrioid and 4 clear cell carcinomas. All the cases examined, including both undifferentiated and well-differentiated carcinomas, showed a positive reaction to this antibody. Staining intensity and the number of positive cells somewhat varied among the cases. Additionally, 2 cases of ovarian epithelial tumors of low malignant potential (I,MP) were stained with this antibody. Both the cases were positive, but the number of positive cells seemed to be rather less than that found in the carcinoma groups. Thus, no differences in ras p21 expression were observed between the cases examined in spite of the differences in the degree of differentiation of the epithelial ovarian carcinomas. However, the possibility remained that the number of positive cells could be an indicator of malignant potential, enabling us to distinguish LMPs from carcinomas.