Measurement of serum C3 does not provide precise informations concerning an eventual consumption of this complement component during an immunological process. An increased synthetic rate may compensate an accelerated catabolism. The study of breakdown products of C3, such as C3d is a more sensitive approach of the role of complement in some immunological disorders. Therefore C3d was measured in the serum of patients with chronic non systemic glomerular diseases. High values of serum C3d were found in all cases of hypocomplementemic glomerulonephritis. Circulating C3d was also increased to a lower extent, in patients with normocomplementemic nephritis such as minimal change disease, mesangial nephritis with IgA deposits and membraneoproliferative (type I) glomerulonephritis. The data suggested the involvement of complement in a number of glomerulonephritis. Participation of complement in immunological disorders particularly in chronic non systemic glomerulonephritis could require a reevaluation when functional tests are performed in addition to static measurements.