Recent studies have described the generation of a mAb, designated DF3-P, which reacts with underglycosylated precursors of the DF3/MUC1 mucin-like glycoprotein. The present work demonstrates that the epitope recognized by mAb DF3-P is expressed by cell lines derived from human epithelial ovarian carcinomas and not a teratocarcinoma. Indirect immunofluorescence assays of single-cell suspensions support expression of the DF3-P epitope on the surface of ovarian carcinomas. Immunofluorescence studies on chamber slides further demonstrate that the mAb DF3-P-reactive cells are present in clusters. We also demonstrate that 125I-labeled mAb DF3-P selectively localizes to human ovarian carcinoma xenografts in athymic mice. The percentage of injected 125I dose/g tissue ranged between 10 and 17% for implanted CAOV-3 and OVCAR-3 tumors. Finally, the results of immunoperoxidase staining studies demonstrate that the DF3-P epitope is detectable in formalin-fixed sections of ovarian tumors and that mAb DF3-P exhibits little if any reactivity with normal surrounding tissues. Selective expression of the DF3-P epitope may be useful as a target for radioimaging or immunotherapeutic approaches to ovarian cancer.