A modification of the conglutinin-binding assay has been used for antigen identification in circulating immune complexes (IC). The complexed antigen was identified, in conglutinin-bound complexes, by antibodies or antibody fragments, specifically directed against the antigen. These antibody preparations were either radiolabelled or detected with 125I-protein A. This system was found efficient using in vitro-formed bovine serum albumin-anti-bovine serum albumin and tetanus toxoid-anti-tetanus toxoid soluble immune complexes at equivalence or at slight antigen or antibody excess. In addition, with 125I-IgG-anti-HBs and with 125I-F(ab')2-anti-HBs, we examined 44 sera from 41 patients with viral type B hepatitis and the presence of HBs was observed in 18 (40.9%) IC containing tested samples. Therefore, this method appears applicable to clinical investigation.