An assay specific for immune complexes containing Entamoeba histolytica antigens has been developed. Rabbit antibody specific for human gamma globulin is attached to a solid phase (nitrocellulose). During the first incubation step polyethylene glycol-precipitated immune complexes are bound to the rabbit antibody attached to the solid phase. In the second incubation step a radiolabelled rabbit antibody specific for Entamoeba histolytica combines with E. histolytica antigens in the complexes. Quantitation of the radiolabel provides a direct measurement of the level of specific immune complexes. The principle of the method has been verified using artificial soluble immune complexes prepared by mixing a pool of serum containing a high titre of anti- E. histolytica antibodies (> 1 : 2048 by indirect hemagglutination assay) with the E. histolytica antigen. This antigen was prepared from axenically cultured NIH : 200 strains of Entamoeba histolytica. Using sera from patients with clinical amoebic disease this method detected immune complexes containing E. histolytica in a high proportion of cases.