Antibodies to oxidized LDL (ox-LDL) and LDL-containing immune complexes (LDL-IC) have been reported to be associated with the presence or progression of arteriosclerosis. We screened for anti-modified LDL antibodies and isolated soluble IC by precipitation with 3.5% (w/v) polyethylene glycol (PEG) 6000 in two groups. The patient group was constituted by 16 insulin- dependent diabetes mellitus subjects free of macrovascular complications. The control group was constituted by 16 healthy, age-, gender-, race-, and body mass index-matched nondiabetic subjects. We detected anti-ox-LDL antibodies and anti-malondialdehyde-modified LDL antibodies with similar levels in patients and controls, while the levels of anti-glycated LDL antibodies were very low, but slightly higher in diabetics than in healthy controls. Isolated LDL-IC were adsorbed to red blood cells (RBC) and incubated with human macrophages for 18 hr at 37°C. Under those experimental conditions, RBC- adsorbed IC are taken up by macrophages but the RBC remain intact and are not ingested. Slightly higher levels of cholesteryl ester (CE) accumulation were measured in macrophages incubated with RBC to which we adsorbed IC isolated from diabetics (15.4 ± 2.5 μg/mg of protein, mean ± SEM) than in macrophages incubated with IC isolated from controls (12.5 ± 1.6 μg/mg of protein, mean ± SEM), but the difference did not reach statistical significance. PEG-precipitable IC isolated from both normal and diabetic subjects led, in some instances, to the transformation of macrophages into foam cells. Significant correlations were observed between CE accumulation and the content of apo B (P < 0.0001), total cholesterol (P = 0.0004), IgG (P = 0.015), and IgA (P = 0.015) in the isolated IC. The correlation between CE accumulation and the content of apo B in isolated IC was stronger in diabetics than in the control group (r = 0.759 vs r = 0.500). Fractionation of isolated IC in immobilized protein A/G yielded immunoglobulin-rich fractions which contained cholesterol and IgG anti-ox-LDL antibodies. The cholesterol content of these fractions was significantly correlated (P = 0.001) with CE accumulation. In conclusion, both diabetics and normal individuals have circulating IC whose atherogenic potential appears to be related to the presence of LDL and antibodies of the IgG and IgA isotypes.