T he low density lipoproteins (LDL) of d 1.019-1.063 g/ml of patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency show marked heterogeneity when viewed with the electron microscope. At least two types of particles are present, one large and the other small. The large particles predominate in a LDL subfraction of large molecular weight isolated by gel filtration on 2% agarose gel. They appear to be flattened structures with diameters mainly in the range of 900-1200 A. The small particles predominate in a LDL subfraction of smaller molecular weight isolated by filtration on the same type of gel. They are 210-250 A in diameter and are similar to normal LDL in size and shape.The high density lipoproteins (HDL) also are heterogeneous. The majority of particles are disc-shaped structures 150-200 A in diameter. The discs are mainly present in stacks which have a periodicity of 50-55 A and a variable length. Each disc appears to be made up of a rosette of smaller globular units 50 A in diameter. The appearance of these large molecular weight HDL contrasts with that of normal HDL, which are 70-100 A in diameter and aggregate in monolayers that show hexagonal packing of particles. A small percentage of the patients' HDL consists of structures 45-60 A in diameter. These predominate in a smaller molecular weight HDL subfraction isolated by gel filtration on Sephadex G200. The particles are present in monolayer aggregates but never form stacked structures similar to those seen in the large molecular weight HDL subfraction.