Monoclonal antibody HEA125 was used to study the tissue distribution of an epithelial cell surface glycoprotein of Mr 34,000 (Egp34). A large panel of normal and neoplastic tissues was examined for immunoreactivity with HEA125 by means of a sensitive immunoperoxidase technique. HEA125 labeled most epithelial cell types throughout the body but did not label any nonepithelial tissue. Major exceptions were epidermal keratinocytes, gastric parietal cells, hepatocytes, thymic cortical epithelial, and myoepithelial cells. Normal mesothelial cells were unreactive. In normal glandular epithelia and tubular adenocarcinomas exclusively the basolateral cell membranes were stained. HEA125 intensely reacted with all tested carcinoma specimens derived from colorectum, stomach, pancreas, liver, lung, mammary gland, ovary, thyroid, kidney, urinary bladder, and prostate including a number of anaplastic, diffusely infiltrating carcinomas. Metastatic lesions of these tumors were consistently positive. Generally, the staining of tumor cells was very homogeneous. The majority of squamous cell carcinomas were less strongly labeled than adenocarcinomas; keratinizing areas of the tumor masses were negative. Germ cell tumors and mesotheliomas of epithelioid type focally expressed the antigen. Egp34 was found to be absent from sarcomas, lymphomas, melanomas, and neurogenic tumors. Hence, HEA125 is a useful reagent for the distinction of carcinomas from nonepithelial neoplasms, even at very low degrees of histological differentiation. Furthermore, HEA125 allows the immunohistochemical detection of micrometastases originating from carcinomas. The antigen is detectable in formalin-fixed paraffin sections.