Complexes formed between apolipoprotein A-I (apo A-I) and dimyristoylphosphatidylcholine (DMPC) or egg phosphatidylchohne have been studied by high-field H NMR, nondenaturing gradient gel electrophoresis, electron microscopy, and gel filtration chromatography. Emphasis has been placed on an analysis of the particle size distribution within the micellar complexes produced at lipid/protein molar ratios of 40-700. As determined by electron microscopy and gel filtration of DMPC/apo A-I complexes, the size of the discoidal micelles produced appears to increase uniformly with an increasing lipid/protein ratio. By electron microscopy, the diameters of isolated DMPC/apo A-I discoidal micelles range from approximately 89 A at a 40 molar ratio to 205 Á at a 700 molar ratio. Analysis of the micellar complexes by ‘H NMR shows that concomitant with the increase in size is the progressive downfield shift of the choline N-methyl proton resonanee of the complex which is observed from 3.245 to 3.267 ppm over the above molar ratio range. The relationship between chemical shift and micelle size is most simply interpreted as arising from a weighted averaging of two lipid environments-lipid-lipid and lipid-protein. In contrast to the above interpretation of the gel filtration expenments on DMPC/apo A-I complexes, nondenaturing gradient gel electrophoresis analysis of particle size distribution leads to an unexpected observation: as the DMPC/apo A-I ratio increases, discrete complexes of increasing size are formed in an apparently quantized manner. A mechanism is proposed for this incremental increase in size that involves a combination of conservative changes in the stoichiometry and conformation of the lipid-associating domains (amphipathic helices) of apo A-I. © 1984, American Chemical Society. All rights reserved.