Anti-native DNA antibodies were studied using an immunoglobulin class-specific enzyme-linked immunosorbent assay (ELISA) in 450 sera, virtually all of which were antinuclear antibody positive. ELISA was positive in about 85% of systemic lupus erythematosus (SLE) sera, usually at high titer and for two or three isotypes. Virtually all sera with antibodies of the three main classes were collected from SLE patients. Very high titers were unique to SLE. In contrast, low-titer antibodies of a single, mostly IgM, class were found in certain patients without evidence of autoimmune disease or with non-SLE autoimmune diseases. The isotypy and titer of the antibodies hence appear to be critical parameters for a correct interpretation of results. Under these conditions, ELISA seems to be usable as single screening test for the demonstration of anti-DNA antibodies.