The DNA base excision repair pathway is responsible for the repair of alkylation and oxidative DNA damage. A crucial step in the base excision repair pathway involves the cleavage of an apurinic/apyrimidinic (AP) site in DNA by an AP endonuclease (APE). The major AP endonuclease in mammalian cells is APE/ref-1, a multifunctional enzyme that acts not only as an AP endonuclease but as a redox-modifying factor for a variety of transcription factors. The purpose of this study was to determine the expression of APE/redox factor-1 (ref-1) in ovarian tissues, particularly ovarian cancers. Formalin-fixed, paraffin-embedded specimens of ovarian tissues (normal,various benign conditions, and epithelial cancers) were studied using both polyclonal and monoclonal antibodies to APE/ref-1. The relationship between APE/ref-1 protein levels and DNA repair activity was studied in ovarian Hey and Hey-C2 cell lines using Western blot and a specific AP-site oligonucleotide cleavage assay. Hey and Hey-C2 cells were fractionated, and the nuclear and cytoplasmic extracts were quantitated for protein levels and assessed for APE/ref-1 with Western blot. Normal ovarian tissues consistently demonstrated strong nuclear staining of the surface epithelium, epithelial inclusions, corpora lutea and albicantia, and stroma. Cytoplasmic staining was absent. A similar pattern was seen for benign conditions including endometriosis. Low malignant potential ovarian cancers stained in a pattern similar to normal ovarian and nonneoplastic tissues; however, two specimens also had areas of cytoplasmic staining. Epithelial ovarian cancers were remarkably different from all other ovarian tissues studied. Both nuclear and cytoplasmic staining of the malignant epithelium were seen and ranged from strong to weak, often with considerable staining heterogeneity within the same tumor. The AP-site oligonucleotide cleavage assay indicated that APE/ref-1 protein levels correlate well with DNA repair activity. The increased levels of APE/ref-1 in the Hey-C2 cells was mainly attributable to increased cytoplasmic enzyme. APE/ref-1 immunoreactivity is altered in malignant ovarian tumors. Further studies will determine whether the altered expression and subcellular location reflect changes in redox regulatory functions.