The conformation of apolipoprotein A-I (apo A-I) was studied in reassembled lipoprotein particles containing various proportions of apo A-I, phospholipids and cholesteryl esters or triglycerides, using both theoretical and experimental approaches. The proportions of the components were chosen to permit the formation of swollen discs mimicking the intermediate particles that must result from the lecithin:cholesterol acyl transferase (LCAT)-mediated transformation of nascent discoidal high-density lipoprotein (HDL) into mature spherical HDL. Three groups of particles were defined by the number of helices per apo A-I molecule that could fit the edge of the swollen discs. Groups I, II and III had six, seven and eight helices per apo A-I, respectively. The conformation of apo A-I was studied by quenching of tryptophan fluorescence in the presence of KI. In all groups the tryptophan (Trp) residues belonging to the helices located on the edge of the particles were very sparingly hydrated. In contrast, the degree of hydration of Trp residues in external position was different in each group. The structural organization of phospholipids was assessed by measurements of the fluorescence anisotropy of 1-(4-trimethylammonium phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) probe. In group I, which contained the less swollen particles, a segment of apo A-I of about 46 amino acids contributed to the area of the faces of the particles, resulting in a highly condensed and ordered phospholipid matrix. In groups II and III, the external segment of apo A-I did not interact with phospholipids.