A human autoimmune serum is used to characterize a protein which is located within a dot-like structure of the interphase nucleus. The dots are located in the nucleoplasma outside the nucleoli. The dot-like structure could be observed on a variety of human substrates (HEp-2, HepG2, HeLa, Molt-4, WI-38 cell lines, peripheral blood lymphocytes), mouse cell lines and tissues (3T3, L929, spleen sections), rat sections, marsupial PtK2 cells and hamster cell lines. The range of the number of dots and their size differed between cells and cell lines and varied between 1 and 24. The antigen could be identified as a 53 kDa protein with a pI of 8.7 and was named NDP53. Digestion experiments suggested that the protein is not associated with DNA or RNA, but is associated with the nuclear matrix. Immunelectron microscopy using ultra-thin sections revealed filamentous structures with a diameter of 0.1 to 0.6 microns. The antigen colocalizes with Sp100, PML and NDP55, which are part of a multiprotein complex known as PML oncogenic domain (POD), nuclear bodies, Kr bodies or ND10.