An immune complex of lipoprotein-antibody was found in human plasma and in tissue liquid of the aorta wall impaired by atherosclerosis. In the majority of cases the antigen component of the immune complex was lipoprotein of very low density. After dissociation of the complex at acid pH, immunoglobulin[Ig]G was isolated, which was identified by double immunodiffusion in agar gel; the Ig reacted in RBS [complement fixation reaction] with autologous and sometimes with isologous lipoproteins.