The interaction of mouse peritoneal and human pericardial macrophages with lipoprotein (LP)-antibody (Ab) immune complex isolated from the serum of ischemic heart disease (IHD) patients has been studied. It is shown that the Ab of the autoimmune complex belongs to IgG class, and the antigen is the LP with d less than 1.063 g/ml. Incubation of mouse peritoneal macrophages with such complex led to the activation of [14C]oleic acid incorporation into cholesteryl esters by 2.5-2.8-fold, in comparison with the experiments where macrophages were incubated with free apoprotein (apo) B-containing LP isolated from the same serum. Incubation of human pericardial macrophages with autologous LP-Ab immune complex led to the transformation of macrophages into foam cells. These data lead to the conclusion that formation of LP-Ab autoimmune complexes may play an important role in the formation of atherosclerotic lesions of the arteries.