Antibodies producing an unusual immunofluorescent pattern were identified in the sera of patients with diverse autoimmune features. This pattern was characterized by the presence of up to six round discrete nuclear bodies in interphase cell nuclei. Immunoblotting analysis showed that these sera recognized an 80-kD nuclear protein, and affinity-purified anti-p80 antibody from the protein band reproduced the fluorescent staining of nuclear bodies. Colloidal gold immunoelectron microscopy showed that the affinity-purified anti-p80 antibody recognized the coiled body, an ultramicroscopic nuclear structure probably first described by the Spanish cytologist Ramon y Cajal. Five cDNA clones were isolated from a MOLT-4 cell lambda gt-11 expression library using human antibody and oligonucleotide probes. The longest cDNA insert was 2.1 kb and had an open reading frame of 405 amino acids. A clone encoding a 14-kD COOH-terminal region of the protein was used for expression of a beta-galactosidase fusion protein. An epitope was present in this COOH-terminal 14-kD region, which was recognized by 18 of 20 sera with anti-p80 reactivity, and affinity-purified antibody from the recombinant protein also reacted in immunofluorescence to show specific staining of the coiled body. This is the first demonstration and molecular cloning of a protein that appears to have particular identification with the coiled body, and it was designated p80-coilin. Autoantibody to p80-coilin may be useful for the elucidation of the structure and function of the coiled body, and the availability of a cDNA sequence could be helpful in further studies to clarify the clinical significance of this autoantibody response.