Immunohistochemical characteristics of undifferentiated carcinomas of the ovary were examined using formalin-fixed, paraffin-embedded tissues with an avidin-biotin staining approach. Eight cases were collected from the pathology files of our Institute from a total of 214 recorded malignant ovarian tumors. For immunostaining, antibodies reacting with epithelial membrane antigen (EMA), pankeratin, vimentin, CA 125, CA 19-9, carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), alpha-1-antitrypsin (AT), epidermal growth factor receptor (EGFR), c-erbB-2, bcl-2 and p53 proteins were used. All the cases examined were positive for EMA and pankeratin, specific markers for epithelial tumors, negative for the non-epithelial tumor marker, vimentin, and also positive for EGFR. Interestingly, only one case was positive for CA 125, despite it being one of the commonest reported indicators of ovarian cancer. CA 19-9 was positive in 7 cases, CEA in 5, AFP in 2, AT in 6 and c-erbB-2 protein in 4. Two cases were positive for p53 protein, and in 1 of these positive staining for bcl-2 was also observed. These results indicate that the epithelial nature is well preserved in undifferentiated ovarian carcinomas, although consistently positive reactions were not observed within the cases for some antigens. They further clearly show that a negative signal for CA 125 can not be considered to exclude the possibility of a primary ovarian tumor.