An enzyme linked immunosorbent assay (ELISA) for C3d bearing circulating immune complex (CIC) was developed.Sera sample were precipitated by plolyethylene glycol(PEG)6000,and the precipitates were added to thg polystyrene micro titer plates which coated with anti human IgG,followed by adding HRP conjugated rabbit anti C3d.The levels of C3d bearing CIC in patients with various diseases as well as the normal controls were measured.It was found that the positive rate of C3d bearing CIC was significantly elevated in patients with SLE,nephrosis,and hepatic diseases.Its clinical significance was also studied.