AbstractUtilizing three different anti‐keratin antibodies and the avidin‐biotin peroxidase complex system on sections of formalin‐fixed paraffin‐embedded tissues and on cryostat sections, immunohistochemical localization of keratin type intermediate filaments in mammary Paget's disease and extramammary Paget's disease was investigated. Anti‐keratin antibodies EAB‐903 and EAB‐904, which recognize 66K and 57K dalton keratin peptides, did not decorate any Paget's cells in either mammary or extramammary Paget's disease. On the other hand, anti‐keratin antibody MAK‐6, which recognizes 52.5K, 50K, 48K, 45K and 40K daltons keratin peptides, did decorate Paget's cells in both Paget's diseases. These staining properties of Paget's cells were the same as those of secretory cells in normal human sweat glands and mammary glands. Anti‐keratin antibody MAK‐6 is thought to be useful in the diagnosis of mammary and extramam