We have studies the secondary structures of the protein moieties of very low density lipoproteins (VLDL) and intermediate density lipoproteins (IDL) of human serum by circular dichroism (CD). Two potential complications in the application of this technique to lipoproteins have been evaluated. First, using chronographic potentiometry in CD measurements of VLDL fractions of different mean particle diameters, we have analyzed statistically the CD signals in order to define the limits imposed by light scattering with respect to both particle diameter and wavelength. We found that CD measurements can be made to as low as 210 nm on particles of 520 A or smaller, and to 194 nm on particles of 450 A and below. Second, we have evaluated the CD contribution of lipid chromophores. Despite the high ratio of lipid to protein, the relative CD effect of the lipids is smaller than for low density lipoproteins (LDL). due to the extremely small ellipticity of natural VLDL triglycerides. Thus, CD measurements can be obtained with confidence on the preponderant bulk of normal VLDL. For the first time we report the CD spectra of human VLDL and IDL. In contrast with human LDL and the lipoproteins of the hypercholesterolemic rabbit, the entire CD SPECTRUM OF HUMAN VLDL shows increased ellipticity with decreasing temperature, which is completely reversible. We have found that the protein moieties of human VLDL and IDL contain substantially more helix (approximately 50%) than does that of human LDL.