A factor of protein nature that forms specific complexes with low density lipoproteins (LDL) has been detected in extracts of aortic intima-media from men and rabbits. Although the formation of complexes is maximal at low ionic strength, it is still observed under physiological conditions. Gel electrophoresis of the complex formed between the extracts and LDL proteins indicates that the insoluble material is an aggregate of the lipoproteins with some of the proteins present in the extracts. Pretreatment of the intima-media extract with proteases greatly diminishes its complexing ability. The interaction was also observed between the extracts and sera. The sera from patients with myocardial infarct formed more insoluble complexes than the sera from a control group. The lipoprotein complexing agent was found in intima-media of aorta and coronary arteries but not in that of veins, pulmonary arteries or in the adventitia of arteries or veins.