The structure of a single bilayer vesicle of dimyristoylphosphatidylcholine has been characterized by sedimentation, densimetry, and light-scattering measurements. The molecular weight, partial specific volume, Stokes radius, and degree by hydration were found to be 2.68 X 10(6), 0.972 cm3/g, 125 A, and 0.86 g/g, respectively. From these quantities, a spherically symmetrical model has been derived that features a phospholipid bilayer 35.5 A thick and a hydration shell 9.3 A thick. This particle was shown to bind apolipoprotein C-III (apoC-III) up to 0.08 g/g without loss of its original vesicular structure. At protein-lipid ratios in excess of 0.08 g/g, sedimentation, gel chromatography, and light-scattering measurement indicated a dramatic decrease in Stokes radius and molecular weight. The sedimentation data showed these parameters to become constant at protein-lipid ratios in excess of 0.25 g/g. In this region, the Stokes radius and molecular weight were found to be approximately 80 A and 442 000, respectively. Within the constraints of these values and other data, several models for this complex are discussed.