Using the Tau-1 (or Tau-13) monoclonal antibody, the distribution of endogenetic Tau in the SH-SY5Y cells, HEK-293 cells and HeLa cells have been observed, and the localization of the indirect immunofluorescent signals in both cytoplasm and nuclei in the interphase have been found. In particular, the fluorescent signals were significantly strong in HeLa nuclei. During cell division (metaphase and anaphase), few Tau-1 signals were observed in the nuclei under the same conditions. To confirm the nucleoplasm localization of Tau, the nuclei of SH-SY5Y cells were isolated and it was observed that Tau-1 signals were co-localized with Hoechst33258 (blue) in nucleoplasm using double immunofluorescence. Western-blotting also exhibited tau protein isoforms localized in both cytoplasm and nucleoplasm. It indicates that Tau protein is localized in tumor cells, and also in their nucleoplasm.