Mr 50 kDa (pI 4.9) intermediate filament polypeptide, keratin K15 (formerly also designated cytokeratin 15), detected by immunofluorescence microscopy in basal cells of stratified epithelia (including epidermis, tongue, esophagus, exocervix) and complex stratified epithelia (e.g. broncheal epithelium), in luminal epithelia of mammary, salivary and sweat gland. In the hair follicle, staining was restricted to the outermost cell layer of the outer root sheath, within the hair follicle several cell layers of the hair follicle bulb were positive with antiserum gp 15.1. In human fetal tissues keratin K15 was localized in basal cells of epidermis, lung, trachea, tracheal glands and esophagus. In fetal hair anlagen, all cells were positive with antiserum gp 15.1, suggesting that keratin K15 is preferentially expressed in epithelial stem cells. Completely unreactive were cells of liver, colon, kidney, urinary bladder, pancreas and lung alveolus. In tumors, gp15.1 is reactive with squamous skin and lung carcinomas and with a significant subgroup of invasive ductal breast carcinomas, especially tumors with higher grade malignancy (Heid, Bartek, Leube, Moll, Kaufmann & Franke, unpublished results).