Conglutinin solid-phase radioimmunoassay (conglutinin RIA) was utilized for detection of soluble immune complexes in sera. Crude conglutinin was obtained from whole bovine serum using Zymosan as an affinity absorbent. The further purification of crude conglutinin was performed through Sephadex G-200 and DEAE-cellulose column chromatography. The activity of purified conglutinin to haemagglutinate EAC3d was found to be more than 1:2048 in titer. Anti-conglutinin antisera raised in rabbit gave an identical precipitin line against bovine serum and purified conglutinin. Conglutinin RIA was performed using 2.5μg/ml of conglutinin and 25μl of fresh normal human serum as a source of complement. Serum samples were incubated with conglutinin coated on polystylene tubes for 1 hour at room temperature, and incubated with (125)I-labelled anti-human IgG for 4 hours at room temperature. The amounts of immune complexes measurable by this method were in the ranges of 5 to 320μg/ml equivalent to aggregated IgG. Conglutinin RIA was applied to the detection of soluble immune complexes formed in vitro which could be detected both in antigen excess and antibody excess regions.