This study demonstrates that in systemic lupus erythematosus (SLE), the presence of immune complexes on the glomerular basement membrane (GEM) does not invariabley result in histological and/or functional lesions of the kidney. Among a group of 29 lupus patients, six subjects were selected for thorough investigation, because their renal function was normal or only slightly altered though they had suffered from SLE for 20 months to 18 years. All patients had antinuclear factor, anti-native-DNA antibody and a low level of complement; 3 had anti-denatured-DNA antibody, 2 had denatured DNA-anti-denatured-DNA circulating complexes and 3 had anti-RNA-protein antibody. Kidney biopsies disclosed either no histological lesion or minimal changes in five of them and diffuse proliferative glomerulonephritis in one. By contrast, using the immunofluorescent technique, granular deposits containing the third component of complement (C3) were found on the GBM of all patients; IgG was present in 5 cases, IgM in 3, fibrinogen in two cases and around the tubules of one. Electron microscopy confirmed the presence of subendothelial and mesangial deposits. Our results also showed a good correlation between the importance of deposits and the presence of denatured DNA-anti-denatured-DNA circulating complexes. From the data obtained in these 6 cases as well as in the 23 other patients of the group, 3 categories of lupus patients could be distinguished with regard to kidney involvement: 1) patients with insignificant histological lesions, no immune deposits and essentially normal function; 2) patients with definite histological lesions, immune deposits and renal insufficiency and 3) patients with few if any histological lesions and little functional impairment contrasting with important immune deposits. The resistance of some patients to the mephrotoxic effects of immune deposits shows that lupus nephritis depends on intricate pathogenic mechanisms and suggests that these are possible antagonized by "protective" factors.