W e studied the metabolism of lipoprotein-proteoglycan complexes by macrophage-derived foam cells. Foam cells were isolated from atherosclerotic rabbit aortas. ApoB-lipoprotein-proteoglycan complex was isolated from human aorta fibrous plaque lesions and LDL-proteoglycan complex was formed in vitro. Both in vitro and in vivo complexes stimulated cholesteryl ester synthesis in foam cells by a dose-dependent, saturable process that resulted in the intracellular accumulation of cholesteryl ester. Stimulation of cholesteryl ester synthesis was linear with time over a 32-h period. Polyinosinic acid inhibited the stimulation of cholesteryl ester synthesis by the complexes by 32-37%, whereas cytochalasin D only produced a 6-16% inhibition. Foam cells degraded 125I-LDL-proteoglycan complex and 125I-acetyl LDL in a saturable, dose-dependent manner. Excess unlabeled acetyl-LDL inhibited the degradation of 125I-LDL-proteoglycan complex by 52%, while LDL had no effect. Similarly, excess unlabeled complex suppressed the degradation of 125I-acetyl-LDL by 48%. Foam cells degraded 125I-methyl-LDL-proteoglycan complex to the same extent as 125I-LDL-proteoglycan complex. These results show that foam cells from atherosclerotic lesions metabolize lipoprotein-proteoglycan complexes predominantly via receptor-mediated endocytosis and consequently continue to accumulate intracellular cholesteryl ester.