L ow density lipoproteins (LDL) and high density lipoproteins (HDL) from the plasma of patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency have been characterized by gel filtration, analytical ultracentrifugation, and gel electrophoresis, and their relative content of lipid and protein has been determined. The LDL of d 1.019-1.063 g/ml show marked heterogeneity. A subfraction of the LDL emerges from columns of 2% agarose gel with the void volume, has corrected flotation rates (S_f°) in the range of 20-400, and contains 4-10 times as much unesterified cholesterol, phosphatidylcholine, and triglyceride per mg protein as normal LDL. A major subfraction of the LDL emerges from the gel in the same general position as normal LDL, but exhibits somewhat higher flotation rates and contains 1.5-3 times as much unesterified cholesterol and phosphatidylcholine and 13 times as much triglyceride per mg protein. The HDL, shown to be heterogeneous in earlier studies, are mainly comprised of molecules which have flotation rates of F_1.20 3-20, migrate in the α₁-α₂ region on electrophoresis, and contain about 12 times as much unesterified cholesterol and 5 times as much phosphatidylcholine per mg protein as normal HDL. Smaller molecules are also detected, which have flotation rates of F_1.20 0-3, migrate in the prealbumin region on electrophoresis, and contain only slightly more unesterified cholesterol and phosphatidylcholine per mg protein than normal HDL.