The Tau-1 monoclonal antibody was localized to the nucleolus of interphase cells and the nucleolar organizing regions (NORs) of acrocentric chromosomes in cultured human cells. Putative nucleolar and NOR tau was found in CG neuroblastoma cells which contain nucleolar tau and little cytoplasmic tau. To further establish the presence of tau in the nucleolus of these cells, sense and anti-sense transfection strategies were used. CG neuroblastoma cells were transfected with tau sense cDNA and immunostained with Tau-1. Cytoplasmic Tau-1 staining was greatly increased in CG cells which contain very little endogenous cytoplasmic tau. Nucleolar Tau-1 staining was also increased in certain CG cells indicating an increase in nucleolar tau in a subset of transfected cells. CG cells were also transfected with tau anti-sense cDNA which abolished Tau-1 staining in the nucleolus. These results contribute to a growing body of evidence defining tau as a multifunctional protein found in both the cytoplasm and nucleoli of primate cells.