The presence of apo E-containing HDL in familial hypercholesterolaemia was investigated and its removal by LDL-apheresis using a dextran sulphate cellulose column was demonstrated by measurement of the apo E/apo A-I molar ratio of serum and by nondenaturing polyacrylamide gel electrophoresis followed by immunoblotting. The molar ratios of apo E/apo A-I in the density greater than 1.063 kg/l fraction of serum obtained from two homozygous patients with familial hypercholesterolaemia were higher (0.021 and 0.030) than that from normal subjects (mean +/- SE 0.011 +/- 0.002) (P less than 0.05). Polyacrylamide gel electrophoresis and immunoblotting showed an increase in apo E-containing HDL similar to HDL2, in the plasma obtained from the homozygous patient with familial hypercholesterolaemia. The increased amounts of apo E-enriched HDL were removed from plasma by adsorption with a dextran-sulphate cellulose column. These results suggested that LDL-apheresis using the dextran-sulphate cellulose column, may cause an increase in the turnover rate of the apo E-containing HDL and thus facilitate cholesterol removal from the peripheral tissues.