CA125 is a high molecular weight mucin type glycoprotein, originally defined by the Oc125 monoclonal antibody (MAb) established by Bast et al. Differentepitopes, co-expressed with the Oc125 epitope on the CA125 antigen, have been used for the development of heterologous assays for determination of the CA125 antigen (2). The CanAg CA125 EIA is based on two mouse monoclonal antibodies, Ov197 and Ov185, directed against two independent epitopes of the protein core of the CA125 antigen (3, 4). Assays for CA125 are frequently used to monitor patients with gynecological malignancies such as epithelial ovarian cancer