The sera from most patients with coronary heart disease concurrent with angiographically documented coronary atherosclerosis were found to contain apolipoprotein B (Apo B), neither apolipoprotein E (Apo E) nor apolipoprotein AI (Apo AI) in the circulating immune complexes precipitated by polyethylene glycol-6000 (PEG). Low density lipoproteins are the major components of cholesterol-containing immune complexes precipitated by PEG. A correlation was established between the serum levels of total cholesterol (r = 0,74; p less than 0.01), and low density lipoprotein (LDL) cholesterol (r = 0.77; p less than 0.01) in patients with coronary heart disease and cholesterol in LDL-containing No relationship was found in healthy individuals. The in vitro increase in LDL levels in most patients with coronary heart disease concurrent with coronary atherosclerosis was not followed by an elevation in LDL-containing immune complexes.