Apoliporotein B (apoB) was measured in buffer-extracted homogenates of grossly normal and artherosclerotic human aortic intima by means of an electroimmunoassay procedure. The apoB values which were expressed as microgram per mg tissue dry weight, varied widely, ranging from 0.34 to 18.45 in normal intima and from 0.8 to 12.5 in fatty fibrous plaques. No consistent differences in apoB content were found between normal intimas from thoracic and abdominal aortic regions. There was a statistically significant positive correlation between the quantity of buffer-extractable apoB in normal regions and the plasma cholesterol and triglyceride concentration. Buffer-extractable apoB values were significantly higher in fatty fibrous plaques than in ulcerated lesions from the same vessel. However, fatty fibrous plaque apoB values were significantly lower than those from grossly normal regions from the same aorta, although the topographical distribution of apoB was more widespread in plaques than in normal regions, as shown by immunofluorescence studies. This apparent discrepancy reflected the incomplete extraction of apoB from plaques as contrasted to normal regions. The relatively loosely bound apoB, extractable by standard buffers, may represent intact low density lipoprotein (LDL) and/or very low density lipoprotein (VLDL), while the tightly bound fraction may represent insoluble complexes of intact lipoproteins within the plaque or delipidated apoB.