BACKGROUND: It has previously been reported that patients with systemic lupus erythematosus (SLE) and glomerulonephritis do not have anti- (deoxyribonucleic acid) DNA antibodies in their urine. This finding was attributed to specific entrapment of anti-DNA antibodies by the immune complexes in the glomerular capillary walls. METHODS: This phenomenon has been re-investigated as part of a study of the use of desoxyribonuclease 1 (DNase 1) to treat lupus nephritis (LN). For this purpose an ELISA was developed for the detection of anti-DNA antibodies in urine. It was found that such an assay was very susceptible to the presence of DNase in urine which destroys the antigen coating the plates and gives rise to false negative results. For this reason, it is essential that all tests for anti-DNA antibodies in the urine are carried out in the presence of EDTA to inhibit the endogenous DNase 1 activity. RESULTS: Using this assay to test the urine from 24 patients with LN and non-selective proteinurea, it was found that they all contained anti-DNA antibodies. The amount of anti-DNA antibodies detected in the urine was compared with that expected by calculations from the anti-DNA antibody titre in the serum and total immunoglobulin levels in serum and in urine. It showed that in 20 patients there was neither specific entrapment nor specific excretion of anti-DNA in urine, only the expected amount of leakage. In only three patients was any appreciable entrapment demonstrated and in only one, any excess excretion. CONCLUSIONS: It is suggested that the failure to detect anti-DNA antibodies in the urine in the previous work was due to failure to inhibit the endogenous urinary DNase. It remains to be determined whether the retention of anti-DNA antibodies or excessive secretion is correlated with clinical phases of LN.