The sera of 88 patient suffering from systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA) containing antinuclear antibodies (ANA) were studied for: I) the complement fixing capacity of these ANAs: II) their immunoglobulin class. The results showed that: I) ANAs bind complement to a significant extent in 2 patients with SLE out of 3, but not in RA; II) the ANAs which bind complement belong partially or totally to the IgG class, while most of the ANAs from SLE not fixing complement belong to other classes; III) the ANAs from RA were most often from the IgM class and, surprisingly, they did not bind complement; IV) the occurrence of IgM ANA in SLE patients seems to be associated with clinical manifestations of arthritis. This double test (identification of ANA class and complement binding capacity), appears to be a valuable aid in the cases where diagnosis between RA and SLE is questionable.