Two monoclonal antibodies, DA7 and DC10, were obtained from fusions of mouse myeloma cells with splenic lymphocytes from mice immunized with human breast cancer cells of PMC 42 line. The indirect immunofluorescence studies performed on established tumor cell lines together with immunoperoxidase staining of normal human tissues showed that the components reacting with the antibodies were cytokeratins. Positive reaction was noted in all epithelia derived cultured cells and in all simple epithelial tissues known to express keratin 18. Immunoblotting performed on various cytoskeletal preparations demonstrated strong staining of a single band with a mobility corresponding to that of cytokeratin 18 (45 kD). The negative immunoperoxidase reaction found in different epithelial tissues of seven animal species suggests that both antibodies are specific for human keratin 18. It was shown that DA7 and DC10 antibodies exhibited strong reaction in paraffin embedded tissues fixed in either methacarn or standard formalin. These characteristics predetermine both antibodies as suitable reagents for the specialized histopathological work.