Antibodies to single-stranded DNA (anti-ssDNA) and double-stranded DNA (anti-dsDNA) were determined in serum and urine of nine patients with systemic lupus erythematosus (SLE) presenting with heavy proteinuria (greater than 300 mg/dl), and the activity of anti-DNA antibodies was compared between paired serum and urine samples in each patient. The results were expressed by the anti-DNA activity (radioimmunoassay units per milliliter) divided by the concentration of IgG (milligrams per milliliter), so as to correct for the difference of IgG concentration. Anti-DNA was detected invariably in the serum of these patients (anti-ssDNA/IgG 4.8 to 27.2, anti-dsDNA 5.6 to 107.8). In contrast, anti-DNA activity was not detectable in urine samples from any of the nine patients. The urine samples from these patients, as well as those from normal individuals, were found to contain ssDNA at levels not significantly different from each other (0.25 +/- 0.15 vs. 0.19 +/- 0.07 microgram/mg creatinine, P greater than 0.1). The failure to detect anti-DNA in the urine of SLE patients, however, was not due to the ssDNA contained in the urine, because no anti-DNA activity was detected even after their urine samples had been digested with deoxyribonuclease. On the basis of these results, anti-DNA in the serum of SLE patients was considered to be entrapped in the kidney, probably owing to its binding with DNA deposited in their glomeruli.