We used enzyme digestion to isolate cells from aortas of Rhesus monkeys (Macaca mulatto) fed on control or atherogenic diets for 19-21 months and subjected them to Metrizamide density gradient centrifugation. A majority of the cells from both control and experimental animals equilibrated in the density range of 1.08-1.12, but an additional population of cells (about 10% of the total) equilibrating at lower densities (p = 1.04-1.07) was present in preparations from diseased vessels. Except for a 10-fold increase in esterified cholesterol in cells from diseased aortas, the high density cells of both groups exhibited similar levels of marker enzymes of organelles [iV-acetyl-/?-glucosaminidase and /?-galactosidase (lysosomes), neutral a-glucosidase (endoplasmic reticulum), 5′-nucleotidase (plasma membrane), and catalase (catalase-bearing particles)] and contained similar amounts of unesterified cholesterol. Electron microscopy revealed the high density cells as smooth muscle cells, with many of those from diseased vessels containing cytoplasmic lipid droplets and lipid-containing lysosomes. The latter were identified by acid phosphatase cytochemistry. The low density cells were enriched in both free and esterified cholesterol, acid hydrolases, and catalase. They appeared morphologically as foam cells heavily laden with lipid, which was present as cytoplasmic lipid droplets and as lipid-containing lysosomes. The cytoplasmic lipid droplets occupied 34-38% and the lipid-filled lysosomes another 18-23% of the cytoplasmic volume of the foam cells. The proportion of total aortic cholesterol associated with cells was found to be 22.1 ± 7% (n = 3) in the control aortas and 17.0 ± 4% (n = 4) in the diseased aortas. Circ Res 46: 520-530, 1980