Significant differences in both specificity and avidity of anti-DNA antibodies were observed in the sera of groups of patients with active systemic lupus erythematosus glomerulonephritis, active systemic lupus erythematosus without nephritis, and in IgG eluates obtained by DNAase digestion of isolated glomeruli from glomerulonephritic kidneys. With methylated albumin-kieselguhr fractionated 3H-HeLa DNA as a source of native or single-strand DNA antigen in a modified Farr assay, an increased level of antibody to native DNA was associated with active systemic lupus erythematosus, particularly active nephritis. The avidity of antinative DNA estimated from plots of the reciprocals of bound and free antigen according to the Sips distribution formula was significanly lower in active glomerulonephritis sera than in sera from patients with active systemic lupus erythematosus without nephritis. However, antinative DNA of uniformly high avidity was found in the glomerular eluates. Avidity of single-strand DNA antibodies did not differ in the various patient groups. The data stronly supprot a major role for high avidity antinative-DNA in DNA/antiDNA immune complex-induced glomerular injury in systemic lupus erythematosus.