Clusterin was purified from human serum by sequential affinity chromatography over IgG-, protein A- and Con A-Sepharose. The protein was ∼ 70 kDa by SDS/PAGE under non-reducing conditions and was resolved into ∼ 35 kDa bands under reducing conditions. The protein reacted with clusterin-specific Mabs in ELISA and in Western blots. Its N-terminal sequences agreed with those published for clusterin. An antiserum specific for clusterin made by the above method detected it in complement membrane attack complexes on rabbit erythrocyte membranes. The interaction of clusterin with IgG was physiologically relevant because it was found to increase the rate of formation of insoluble immune complexes. © 1991.