P59Nc is a polypeptide associated with bundles of cytoplasmic and nuclear filamentous structures of 8-10 nm of diameter in Neurospora crassa cells. It is immunologically unrelated to both higher and lower eucaryotic tubulin and actin proteins and is detected weakly by the anti IFA monoclonal antibody. We analyze here the immunological relationship between P59Nc and intermediate filament (IF) mammalian proteins by using anti P59Nc, anti keratin, anti vimentin and anti IFA antibodies. Anti P59Nc antibodies detected a 59 kDa polypeptide from rat and bovine tissues which copurifies with polypeptides of the IF family. Neither P59Nc nor the 59 kDa rat polypeptide were recognized by anti keratin or anti vimentin antibodies. Immunostaining of rat tongue sections with anti P59Nc antibodies showed that the 59 kDa rat polypeptide is present in the cortical cytoplasm of suprabasal epithelial cells. The results indicate that P59Nc shares common antigenic determinants with a still uncharacterized 59 kDa polypeptide from mammalian tissues which have extractability and immunological properties similar to those of IF polypeptides and shows a tissular distribution and a cellular localization similar but distinguishable from keratins.