CA 125 has been extensively evaluated as a serum marker for moni toring patients with epithelial ovarian carcinoma. Recently, consideration has been given to the use of CA 125 as one component in a strategy for early detection of this disease. A number of benign conditions can, however, increase CA 125 in serum, limiting the utility of a single antigen determination for identifying ovarian cancer patients. Coexpression of different epitopes on the high molecular weight complexes that express CA 125 determinants might provide a more specific test for malignant disease, provided that adequate sensitivity were maintained. To determine how frequently determinants are coexpressed, macromolecular moieties containing CA 125 determinants have been isolated from ascites fluid of ovarian cancer patients by immunoaffinity chromatography. CA 125* moieties have been probed on Western transfers Â«Â¡lsheveral murine monoclonal antibodies that recognize distinct tumor-associated epitopes. Marked heterogeneity was observed between patients with regard to antigenic determinants that could be coexpressed with CA 125. A fraction of ascites fluids from different ovarian cancer patients contained moieties which bound to OC 125 on a solid phase immunoabsorbant and which also bound 12'l-labeled monoclonal antibodies NS 19-9, B72.3, DF3, or the novel murine monoclonal antibody OC 3632 in a double determinant immunoradiometric assay. Serum samples were evaluated from patients with ovarian cancer and from apparently healthy individuals. Coexpres sion of TAG 72 and CA 125 was observed most frequently. When the double determinant assay for Coexpression of TAG 72 and CA 125 was compared to assays for the individual antigens, the assay for Coexpression was substantially less sensitive than those for the individual markers. © 1991, American Association for Cancer Research. All rights reserved.