Malignant ovarian tumors may represent either primary ovarian cancers or metastatic lesions (from patients with demonstrated primary cancers at other body sites) whose distinction may be difficult using clinical, surgical, and pathological criteria. Monoclonal antibody (MAb) COL-4, reactive with carcinoembryonic antigen, has previously been shown to react preferentially with adenocarcinomas of the colon versus a variety of normal tissues. We report here that MAb COL-4 is strongly reactive with primary colonic carcinomas (N = 50), as well as regional (N = 42), and distant (N = 20) metastases of colonic adenocarcinoma. In contrast, MAb COL-4 demonstrated little to no reactivity with primary (N = 53) and metastatic carcinomas of the ovary (N = 23) including serous, mucinous, and poorly differentiated adenocarcinomas using immunohistochemical techniques. This differential reactivity was statistically significant (P less than 0.001), suggesting the potential clinical utility of MAb COL-4 in the differentiation of ovarian from colonic adenocarcinoma. Solid-phase quantitative radioimmunoassays and Western blotting techniques confirmed these results. Data are also presented that the carcinoembryonic antigen molecules or epitopes recognized by a more classical broadly reactive anti-carcinoembryonic antigen MAb are distinct from those recognized by MAb COL-4. Other carcinomas which also metastasize to the ovary and may be confused clinically with a primary ovarian tumor such as adenocarcinomas of the stomach and breast were also evaluated for reactivity with MAb COL-4. COL-4 was also reactive with all gastric carcinomas evaluated, but failed to react with breast carcinomas. Hence, COL-4 can now be utilized as an immunohistochemical adjunct for the differentiation of ovarian from gastrointestinal adenocarcinoma which can be difficult to distinguish by clinical, surgical, and histological parameters.