Antinuclear antibodies were detected in 30 of 38 (79%) patients with scleroderma by indirect immunofluorescence using rat liver sections as a substrate. Seventeen of 30 patients who were positive for antinuclear antibodies also had antinucleolar antibodies.Antibodies to non-histone nuclear antigens were identified by double immunodiffusion in 0.45% agarose. The incidence of anti RNP antibody was 13%; anti SS-A antibody, 45%; anti SS-B antibody, 13%; anti Sm antibody, 0%; anti MU antibody, 0%; anti Scl-l antibody, 58%.Scl-1 antigen was not found in isotonic saline soluble extracts of human spleen, human liver, rabbit thymus, rat liver or Raji cell. Alternatively, the antigen was extracted from these organs with hypertonic salt solutions (0.3M, 0.6M, 1.0M, NaCl). The antigen was inactivated by heating at 37•Ž for 1 hour. It was resistant to digestion with deoxyribonuclease or ribonuclease, and was destroyed by trypsin digestion.Antibodies to Scl-1 antigen were detected in sera from 22 of 38 (58%) patients with scleroderma, 1 of 10 (10%) with sclerodermatomyositis, 1 of 26 (4%) with mixed connective tissue disease and 1 of 32 (3%) with undifferentiated connective tissue disease having anti RNP antibody. Anti Scl-1 antibody was not present in sera from 18 patients with polymyositis or dermatomyositis, 88 with systemic lupus erythematosus, 55 with rheumatoid arthritis, 44 with sicca complex or 24 with Hashimoto's thyroiditis. The patients with scleroderma had a significantly high incidence of the antibody (P<0.01). The present study suggests that anti Scl-1 antibody might be a useful marker for scleroderma.