The BC2 monoclonal antibody, which binds to an epitope on the peptide backbone of polymorphic epithelial mucin, was tested immunohistochemically for reactivity with epithelial ovarian carcinoma. This epitope was expressed in 90 of 91 malignant ovarian tumors; in 88% of these, more than 50% of the tumor cells expressed the epitope. In 94% of the positive tumors, the epitope was expressed on the cell membrane; in 56%, cytoplasmic expression was evident; and in 39%, secreted extracellular antigen was detected. Differences were not clearly discernible between dissimilar histotypes with respect to the percentage of cells expressing antigen and antigen localization. Thirteen of 19 benign ovarian cystadenomas also expressed the epitope, but staining was weak and restricted to the luminal surface of the cell membrane. A blind retrospective immunohistochemical analysis of all second-look laparotomy biopsy specimens from 20 patients also was performed. All four patients in whom microscopic disease was detected by standard pathologic assessment had BC2-positive metastases. Of seven patients in whom recurrent disease subsequently developed despite negative pathologic findings, four had biopsy specimens containing BC2 antigen-positive adenocarcinoma-like cells. Of the nine patients with negative results on operation and no recurrence, one had biopsy specimens containing BC2 antigen-positive adenocarcinoma-like cells. Mesothelial cells, although typically negative, expressed the epitope in one biopsy specimen, necessitating caution in the interpretation of positive cells. The BC2 antibody is reactive with most epithelial ovarian carcinomas and appears to be a useful tool for the detection of micrometastases.