Small angle X-ray scattering diagrams of human serum low density lipoprotein (LDL) were recorded at several temperatures in solutions of different freezing points. Modifications of the X-ray patterns observed on cooling the lipoprotein samples below 0.degree. C are due to reversible alterations of the LDL surface structure induced by the freezing process (independent of temperature). With both intact and partially dehydrated LDL, differential scanning calorimetry (DSC) carried out in the body temperature range revealed a heat absorption characteristic of the transition from a liquid crystal to an isotropic liquid phase of cholesteryl esters within the lipoproteins. Small angle X-ray scattering diagrams recorded with the same LDL sample before and after the partial removal of water were very different; the scattering curve for intact LDL showed a strong band centered at (36 .ANG.)-1 which disappeared upon drying and reappeared upon restoring the water. The presence of this signal probably strongly depends on the molecular structure of the lipoprotein surface.