Several methods have been recently developed for the preparation of soluble apo-low-density lipoprotein (apoLLL) Delipidation of LDL alters the immunochemical activity of the molecule and causes an apparent increase in random structure. Despite these changes, soluble apoLDL retains prominent immunological and optical characteristics of native LDL. The lipid of LDL appears to stabilize the protein conformation of LDL derivatives. Optical measurements suggest that native LDL contains a significant amount of pleatedsheet, antiparallel chain beta-structure in addition to random and probably some helical structure, while HDL, by optical criteria, is relatively richer in the alpha-helical conformation. Two proteins containing C-terminal glutamine and C-terminal threonine have been isolated from high-density lipoprotein (HDL). Circular dichroic measurements and total amino acid content are consistent with a greater helical content of apoHDL-Thr than apoHDL-Gln. The techniques of nuclear magnetic resonance and electron spin resonance have been used to probe the structure of LDL and HDL. The presence of protein does not seem to exert a constraining effect on the proton resonance of lipoprotein lipids, while the presence of lipid does appreciably constrain nitroxide tumbling in spin-labeled lipoprotein protein, the constraint being relatively greater in HDL than in LDL. Existing structural evidence is consistent with lipoprotein models in which protein and phospholipid occupy the surface and other lipids are more internal.