Immune complexes consisting of human [125I]LDL or [125I]VLDL and anti-apo B IgG were prepared in vitro. After intravenous administration of these complexes or free LDL to normal rabbits the elimination rate for complexes was 2-3-fold higher than for free lipoproteins. The liver/spleen radioactivity ratio after administration of immune complexes was 34% less than after administration of free lipoproteins. Investigations carried out on the cell cultures have shown that during 4-h incubation the uptake of [125I]LDL-anti-apo B IgG complex by human lung fibroblasts was lower than uptake of free [125I]LDL, whereas mouse peritoneal macrophages took up immune complex more actively than free LDL. During 3 days of incubation of macrophages with LDL-anti-apo B IgG the transformation of macrophages into foam cells was observed. This process was accompanied by almost a 60-fold increase of cholesteryl ester content in the cell. It is suggested that excessive uptake of lipoprotein-antibody complexes by macrophages leading to formation of foam cells may play an important role in atherogenesis.