The lipid and protein composition of very low density lipoprotein (VLDL) and low density lipoprotein (LDL) subfractions (Sf greater than 100, 60--100 and 20--60 VLDL and Sf 10.4--20, 5.7--12 and 3.5--6.5 LDL) in six subjects with type III hyperlipoproteinaemia (HLP) was compared to that of 12 normal subjects. In type III HLP all VLDL subfractions contained increased concentrations of cholesterol and triglycerides and were relatively enriched in cholesterol. VLDL of Sf 20--60 also contained and increased concentration of B-protein. The tetramethylurea (TMU) soluble apolipoproteins of the VLDL subfractions were separated by polyacrylamide disc gel electrophoresis. In the subjects with type III HLP the proportion of arginine rich protein (ARP) was increased in all subfractions. The concentrations of cholesterol and triglycerides were increased in the LDL subfraction of Sf 10.4--20 and cholesterol was decreased in LDL of Sf 5.7--12, but the ratios of cholesterol to triglycerides were not significantly different from those in the LDL subfractions of the normal subjects and the protein composition was also similar. These results provide further evidence that in type III HLP abnormalities are not confined to the stage of conversion of VLDL to LDL, but occur throughout the VLDL spectrum.