DNA molecules were revealed in the glomerular wall of lupus nephritis patients by applying two specific colloidal gold cytochemical approaches at the electron microscope level: immunocytochemistry using a monoclonal anti-DNA antibody in conjunction with protein A-gold and enzyme-gold cytochemistry using DNAse-gold complexes. Application of both techniques has demonstrated that DNA molecules are preferentially located over the electron-dense deposits found in the glomerular basement membrane and mesangial matrix of SLE patients, as well as over the nuclei. Their distribution within the glomerular wall was correlated with electron-dense immune deposits revealed by anti-light chain antibodies. In normal control kidney, DNA labeling was restricted to the cell nuclei. Several control experiments have demonstrated the high specificity of the results. These data thus suggest a possible role for DNA as an antigenic component in the formation of immune complexes.