The prevalence of different antinuclear antibodies was examined in systemic lupus erythematosus, rheumatoid arthritis, Sjögren's syndrome, progressive systemic sclerosis, dermatomyositis, discoid lupus erythematosus, and mixed connective tissue disease. Antibody to Sm antigen (a nuclear acidic protein) was found almost exclusively in serums of patients with systemic lupus erythematosus, suggesting that this antibody might be a "marker" antibody for this disease. However, antibodies to native DNA and to nuclear ribonucleoprotein were found in a variety of systemic rheumatic diseases. In systemic lupus erythematosus, antibody to native DNA was present in high titer but in other rheumatic diseases in low titer. Antibody to nuclear ribonucleoprotein occurred in certain other systemic rheumatic diseases besides the mixed connective tissue disease syndrome. These studies showed that there were distinct profiles of antinuclear antibodies in certain systemic rheumatic diseases, characterized by the presence or absence of certain antibodies and by differences in their mean titers.