Subfractions of CLDL (VLDL), Sf 100-400; CLDL2, Sf 60--100; VLDL3, Sf 20--60) and LDL (LDL), Sf 12--20; LDL2, Sf 6--12; LDL3, Sf 3--6) were isolated from the plasma of three normal, three type III and four type IV hyperlipoproteinemic subjects. In the type IV group, all VLDL subspecies were of normal composition but were increased in concentration in the order VLDL1 greater than VLDL2 greater than VLDL3. In the same subjects, although LDL2 was lowered and LDL3 increased, the total plasma LDL concentration was normal. All VLDL subfractions were elevated in the type III group, but in this case VLDL3 predominated. These subfractions were enriched in cholesteryl esters and depleted in triglyceride. In the LDL density range there was a shift of mass towards the least dense fraction, LDL1, which was of normal composition. EPR studies of the VLDL and LDL subfractions in a type IV subject demonstrated a decrease in fluidity with increasing density. The major change occurred between VLDL3 and LDL1 and was attributed to a substantial alteration in the cholesteryl ester : triglyceride ratio in the particle. A similar argument was used to explain thction in normal or type IV subjects. Particle diameters, determined by laser light-scattering spectroscopy were in good agreement with the values obtained by electron microscopy. This study provides a baseline for the examination of the relationship between the physical and metabolic properties of VLDL and LDL subfractions in type III and IV hyperlipoproteinemia.