The complement fixing activity of antibody to native DNA (CF-n-DNA antibody) was determined in sera of 93 patients with systemic lupus erythematosus (SLE) by complement fluorescent technique and the following results were obtained. 1. The sera of positive CF-n-DNA antibody had significantly higher n-DNA binding and anti complementary activity, lower complement level and mixed type cryoglobulines. 2. The clinical activity of lupus nephritis was correlated with the frequency of CF-n-DNA antibody. Serial studies of 3 patients with lupus nephritis showed a close correlation between the presence of CF-n-DNA antibody and activity of renal disease. 3. The CF-n-DNA antibody was frequently found in patients who had massive deposition of immune complex along the glomerular basement membrane. 4. Sixteen sera of positive CF-n-DNA antibody were treated by DNase. No significant change in the titer of the antibody was obtained before and after DNase treatment. These date suggest that CF-n-DNA antibody was play an important role of pathogenesis in lupus nephritis.