Based on previous studies of the properties of moxestrol, we hypothesized that a radiohalogenated analog of moxestrol, [125I]11 beta-methoxy-17 alpha-iodovinyl-estradiol [( 125I] MIVE2), should bind to the estrogen receptor (ER) in some ovarian adenocarcinomas (OVCA), thereby offering the potential for imaging and/or treatment of these cancers. We used monoclonal antibodies (H222, H226, and D547) against human breast cancer ER to identify the [125I]MIVE2-binding moiety in OVCA cytosols that is found on high salt sucrose gradients. After gel electrophoresis and western blotting, exposure of OVCA extracts to the ER antibodies, followed by exposure to goat antirat serum and then rat peroxidase antiperoxidase, demonstrated a moiety in OVCA that migrated indistinguishably from the ER in MCF-7 human breast cancer cells and from that in specimens of breast cancer tissue. Because few studies have demonstrated efficacy of hormone management for OVCA, we also wanted to learn whether ER exists in multiple forms in OVCA, in view of the possibility that some forms may be inactive in regulating growth-dependent cell functions while retaining estrogen-binding capacity. By incubating the monoclonal antibodies H222, H226, and D547, each of which recognizes a different region on the ER protein, with OVCA cytosol fractions, we demonstrated that ER in OVCA can exist in multiple forms, some of which fail to express an H226-recognized site and some of which fail to express a D547-recognized site. This observation indicates that a relationship may exist between the presence or absence of certain forms of ER in ovarian epithelial cancer and a patient's response to hormone therapy.