Monoclonal antibodies were prepared against native human ovarian carcinoma cells derived from the ascitic fluid of a patient (BH). One antibody, HC7R7, was selected on the basis of its binding to tumor BH cells, to other ovarian tumor cell lines (CAOV-3 and GZL-8), but not to the patient's fibroblasts. One hundred cell smears from ascitic and pleural effusions of tumor-suspected patients were immunostained with HC7R7. All serous ovarian carcinomas and half of the breast carcinomas stained positive with HC7R7; cells from noncancer ovarian aspirates were negative. Mesothelial cells were also stained with HC7R7. A correlation was noted between HC7R7 and OC-125 staining of ovarian tumor cells but not between HC7R7 and c-neu staining of breast tumor paraffin sections. The location of HC7R7-positive material in ovarian tumor cell lines (CAOV-3 and GZL-8) differed from that in the breast tumor cell line (MCF-7). CAOV-3 and GZL-8 showed membrane binding while, in MCF-7, not fully identified intracellular organelles were stained. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and immunoblots from membrane and cytosol fractions of GZL-8 and MCF-7 showed HC7R7 binding to three protein bands in the membrane fraction and to three other bands in the cytosol, all in the 29- to 68-kDa range. Two of the bands were glycoproteins. The only band that was different in the GZL-8 and MCF-7 cells was a 43-kDa glycoprotein, which was more pronounced in the MCF-7 cells. The possible significance of the new HC7R7 antibodies for detection and survey of ovarian malignancies is discussed.