The concentration of LDL-protein (apo B) was determined by electroimmunoassay in the grossly normal intima of 15 human aortas obtained at autopsy. The mean apo B concentration was: 1.02 ± 0.05 SEM (range 0.16 – 2.51) mg per cm³ tissue, or higher than literature values of plasma apo B. No detectable amounts of apo B were found in the neighboring tunica media in any of the cases. In one case apo B concentrations were also measured in tissues from liver, lung, spleen, kidney, brain, myocardium and skeletal muscle, but all had values at least ten-fold lower than aortic intima, except liver (one-fourth intima value). The mean concentrations of human serum albumin (HSA) in the intima was 13.52 mg/cm³, or only one-fourth plasma concentration. Thus the aortic intima not only has the highest apo B values of the tissues tested, but in the intima apo B is retained to a greater degree than another plasma macromolecule. These results may be relevant to the fact that arterial intima is the primary site of atherogenesis.