Antibodies from 5 patients with systemic sclerosis reacted with an antigen localized to the metaphase chromatin, the cleavage furrow and the midbody of anaphase and telophase HEp-2 cells. The titer of antimidbody antibodies ranged from 1:160 to 1:1280. Four patients had systemic sclerosis and one had idiopathic Raynaud's phenomenon. In situ biochemical characterization of the antigen revealed that it was resistant to DNase I, micrococcal nuclease and RNase A, but was sensitive to trypsin treatment. The antigen remained insoluble in 400 mM acetic acid but was extracted from the cells with 400 mM hydrochloric acid. The antibody was not seen in sera from 2500 normal female blood donors, 120 patients with systemic lupus, 60 patients with rheumatoid arthritis, 15 patients with linear scleroderma or 25 patients with Raynaud's disease.