An immunoperoxidase study, using the Avidin-Biotin-Peroxidase complex method and the monoclonal antibodies, anti-carcinoembryonic antigen (CEA) and anti-carbohydrate determinant 19–9 (CA 19–9), was carried out on 108 common epithelial tumors of the ovary and 13 epithelial tumors metastatic to the ovary. Primary mucinous tumors were positive in 62% of the cases (benign, 15%; borderline, 80%; and carcinomatous, 100%) with anti-CEA. None of the serous tumors were positive with anti-CEA, but 27% (benign, 23%; borderline, 40%; and carcinomatous, 20%) were positive with anti-CA 19–9. With anti-CEA, 30% of the endometrioid carcinomas, 50% of the malignant mesodermal mixed tumors, 14% of the clear cell carcinomas, 36% of the Brenner tumors, and 83% of the metastatic carcinomas from the large intestine were positive. With anti-CA 19–9, 76% of the mucinous, 40% of the endometrioid, 25% of the malignant mesodermal mixed tumors, 57% of the clear cell carcinomas, 45% of the Brenner tumors, and all the metastatic carcinomas from the large intestine were positive. All the undifferentiated carcinomas were unreactive with both antibodies. Although neither CEA nor CA 19–9 is a specific marker for any type of ovarian tumor or for malignancy per se, the presence of the former antigen can be useful in differentiating serous from mucinous tumors. Moreover, demonstration of either antigen in a variety of tumors may indicate its potential value as a serum marker in monitoring the course of the patient.