Monoclonal antibody (RCK 105) directed against keratin 7 was obtained after immunization of BALB/c mice with cytoskeletal preparations from T24 cells and characterized by one- (1D) and two-dimensional (2D) immunoblotting. In cultured epithelial cells, known from gel electrophoretic studies to contain keratin 7, this antibody gives a typical keratin intermediate filament staining pattern, comparable to that obtained with polyclonal rabbit antisera to skin keratins or with other monoclonal antibodies, recognizing for example keratins 5 and 8 or keratin 18. Using RCK 105, the distribution of keratin 7 throughout human epithelial tissues was examined and correlated with expression patterns of other keratins. Keratin 7 was found to occur in the columnar and glandular epithelium of the lung, cervix, breast, in bile ducts, collecting ducts in the kidney and in mesothelium, but to be absent from gastrointestinal epithelium, hepatocytes, proximal and distal tubules of the kidney and myoepithelium. Nor could it be detected in the stratified epithelia of the skin, tongue, esophagus, or cervix but strongly stained all cell layers of the urinary bladder transitional epithelium. When applied to carcinomas derived from these different tissue types it became obvious that an antibody to keratin 7 may allow an immunohistochemical distinction between certain types of adenocarcinomas.