Merokeratin is an easily available soluble proteolytic derivative of mature .alpha.-keratin. Guinea pig antibodies were raised to merokeratin prepared from sheep wool. These antibodies decorate in immunofluorescence microscopy arrays of bundles of intermediate sized filaments present in established epithelial cell lines growing in culture. The highly helical soluble proteolytic fragment of mature .alpha.-keratin contains antigenic determinants shared by the cytokeratins present in nonepidermal epithelial cells. Antibodies to these keratin fragments can be used for the demonstration of at least some cytokeratin-containing structures in other cells and tissues.