1. NMR spectra of porcine high- and low density lipoproteins (d 1.120--1.210 and 1.019--1.070, respectively) and their extracted lipids were obtained as functions of temperature, frequency and solution viscosity, and from solutions to which paramagnetic species had been added. 2. About one-third of the N(CH3)3 groups in low-density lipoproteins are so immobile that they do not give a sharp resonance at any temperature up to 65 degrees C, unless the particles are disrupted with sodium dodecylsulphate. Most of the protein residues also undergo little segmental motion. 3. A marked restriction of motion of acyl chain terminal CH3 groups suggests that chain interdigitation occurs in low-density lipoprotein. Apart from this, there is a general ordering of the lipids without a decrease in the rate of rotation about bonds, suggesting that the protein organizes the lipids by controlling the molecular packing rather than by direct strong interactions. The lipids are more ordered in low-density than in high-density lipoprotein. 4. All phospholipids with mobile N(CH3)3 groups are at the particle surfaces, in patches separated by protein. In low-density lipoprotein the patches are raised proud of the protein, whereas in high-density lipoproteins the protein and lipid polar groups are coplanar. 5. The high-density lipoprotein results are consistent with literature models for the structure. The low-density lipoprotein results suggest a new model, which is basically a trilayer. The centre consists of a monolayer of phospholipid with tightly-packed polar groups in contact with a protein core. The outer monolayer of phospholipid contains the rest (most) of the protein; the central layer contains the neutral lipid (cholesterol esters and triglycerides), interdigitated into both the inner and outer monolayers. Unesterified cholesterol is distributed through all three layers.