Mouse monoclonal antibodies against the human mammary carcinoma cell line, MCF-7, were produced and tested against a panel of cell lines. They recognize a variety of intra- and extracellular antigens. Four of them did not react with the human rhabdomyosarcoma cell line A-204 used as a discriminating second target during initial screening. One of these, 45-B/B3, is specific for epithelia, and probably recognizes a cytokeratin. This antibody clearly differentiates carcinomas from sarcomas and lymphomas, and epithelial cells in culture from fibroblasts. It is not species-restricted.