32 patients (22 biopsed) with lupus nephritis (LN) were observed for circulating immune complexes (IC). Solid phase Clq (SPClq) and polyethylene glycol (PEG) precipitation tests were used. The patients were studied during the clinical follow-up in different phases of disease activity. Comparative studies between each histological class of LN and corresponding forms of idiopathic glomerulonephritis (IGN) were made: no significant differences were found between either mesangial LN and stalk mesangial IGN, or between focal proliferative LN an focal proliferative IGN. However, a significant difference was found for SPClq data between diffuse proliferative LN and mesangiocapillary IGN, and between membranous LN and membranous IGN. LN, with an acute nephritic syndrome and hypocomple-mentemia, displayed SPClq data significantly above the levels of IC found in IGN with similar clinical features. IC serum data would seem an important element for the diagnosis and the clinical management of patients affected by LN.