Abstract 1. (1) Using polyspecific antiserum to whole human serum and two-dimensional electro-immunodiffusion, it has been established that a wide range of serum proteins present in arterial intima can be mobilised just as effectively from intact as from minced intimal samples. 2. (2) Using monospecific antiserum to total low density lipoproteins (TLDL) reactive with apolipoprotein B, or antiserum to albumin, and single-dimensional (rocket) electro-immunodiffusion, it has been shown that quantitation of TLDL or albumin respectively can be carried out on the extractable fractions of these proteins from intact intima from the aorta, coronary and pulmonary arteries. 3. (3) In addition to this “labile” fraction, firmly “bound” TLDL is demonstrable in some intimal samples by immunohistology after electrophoresis of the tissue. 4. (4) Variation is found between different sites in the aorta of the same individual in the “labile” TLDL fraction: mean intimal TLDL values for this fraction are found to correlate poorly with serum cholesterol estimations made during life. 5. (5) In areas of aortic intima grossly unaffected by atherosclerotic lesions in subjects aged 40 or over, lipid and lipoprotein may be present and is largely extractable. In atherosclerotic areas the extractable (labile) fraction may be less than in unaffected areas but a significant “bound” fraction is invariably demonstrable. It is therefore proposed that “bound” TLDL is of greater significance than the “labile” fraction in atherogenesis.