D. Atkinson,* A. R. Tall, D. M. Small, and R. W. Mahley* 1 abstract: The plasma lipoprotein HDLC from miniature swine fed a high-cholesterol, saturated-fat diet exhibits a thermal transition (temperature range 25-45 C) of its core-located cholesterol esters. This transition from an ordered, smectic-like structure to a more disordered structure is similar to that described for human plasma low-densitylipoprotein (LDL). Small-angle X-ray scattering measurements demon-strate that HDLC is a spherical particle (~ 180-Á diameter) intermediate in size between human LDL (~ 220 Á) and nor-mal high-density lipoprotein (~ 100 Á). The electron-density profile of HDLC below the transition (10 C) exhibits a single core-located electron-density peak associated with a region of overlapping steroid moieties of the cholesterol esters arranged in a layered structure in the particle core. This electron-density