Studies with human autoimmune sera identified auto-antibodies reacting with a novel antigen of 80 kDa. In interphase mammalian cells, the 80-kDa antigen was enriched in nuclear coiled bodies and was used as a marker for this nuclear structure. This antigen was subsequently named p80-coilin. By light and electron microscopic immunocytochemistry, a number of other antigens were also localized to the coiled body, including components of small nuclear ribonucleoproteins which are involved in the processing of nucleolar and extranucleolar RNA. Although the function of the coiled body is unknown, the presence of these subcellular particles might indicate an involvement in RNA metabolism. The identification of a protein highly enriched in this structure and the availability of specific antibodies might help in its isolation and the study of its function.