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The detailed molecular structure of the complex formed by the apoprotein from porcine high density lipoprotein and dimyristoyl phosphatidylcholine (lecithin) has been investigated by a range of physical techniques. The complex, an oblate ellipsoid with major axis 11.0 nm and minor axis 5.5 nm (see the accompanying paper), is comprised of a section of lecithin bilayer with apoprotein at the surface. The main site of interaction between protein and lipid is in the lipid glycerophosphorytcholine group region; as with native high density lipoprotein the surface of the particle consists of a mosaic of lecithin polar groups and protein. The formation of this mosaic reduces the cooperativity of the lecithin chain motions and changes the curvature of the lipid-water interface, as compared to a bilayer. Otherwise, there are no major changes in lecithin motions indicating that no strong binding of lipid to protein occurs. The interaction involves the intercalation of amphipathic, 60 % α-helical, apoprotein molecules among the lecithin molecules so that the protein resides at the lipid-water interface. The apoprotein has a high affinity for the lipid-water interface but specific lipid-protein interactions are not involved. [ABSTRACT FROM AUTHOR]