A new protein antigen of the nucleolus organizer region (NOR), designated He, was recognized by human autoantibodies obtained from a patient with Raynaud phenomenon. In mitotic cells of all vertebrate species tested. He serum selectively immunostained the chromosomal NORs. A completely unexpected characteristic of the He antigen was its location during interphase. In mammalian cell substrates, it was concentrated in numerous nucleoplasmic granules, with minor amounts of the antigen uniformly distributed throughout the entire nucleus. In interphase nuclei of lower vertebrate cells, however, the antigen was preferentially located in the nucleolus. The antigenicity of He is not dependent on RNA or DNA; its cytochemical properties operationally classify it as a nonhistone component of the chromosome scaffold. The He antigen was present in the residual nucleolar structures of cells that were not at all active in rRNA synthesis, such as mammalian late spermatids and amphibian erythrocytes.