Clones EP17 and EP30 recognize cytokeratins (CK) 8 and 18, both of which are low molecular weight intermediate filament proteins (52.5 kDa and 45 kDa respectively). In normal tissues, CK8/18 recognizes all simple and glandular epithelium (1). In neoplastic tissues, CK8/18 may prove useful for the identification of most adenocarcinomas and some squamous cell carcinomas. CK8/18 expression patterns may also aid in the classification of tumors of unknown origin and poorly differentiated carcinomas (1-3). CK8/18 rabbit monoclonal may be useful as a staining mask in a multiplex stain with mouse monoclonal antibodies such as Ki-67, PD-1 or CD8. As CK8/18 does not stain stromal components or tumor infiltrating lymphocytes in various carcinomas such as breast or colon cancers, a CK8/18 staining mask may be helpful with these applications.