[en] Discoidal lipid particles were prepared from a reaction mixture containing apo A-IV and dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) in the molar ratio of 185:1 (lipid/protein). The complexes were isolated by gel filtration and characterized in terms of composition and size. Infrared attenuated total reflection spectroscopy was used to estimate the secondary structure of apolipoprotein A-IV and the orientation of its amphipathic alpha-helices with respect to the lipid hydrocarbon chains. In addition, infrared spectra were analyzed in terms of the conformation and organization of different regions of the lipid molecules in the particles. This approach has been applied successfully to reconstituted HDL particles prepared from a reaction mixture containing DPPC and apo A-I in the molar ratio of 150:1 (Wald, J.H., Goormaghtigh, E., De Meutter, J., Ruysschaert, J.M. and Jonas, A. (1990) J. Biol. Chem. 265, 20044-20050). Apo A-IV helicity increased for the protein bound to DMPC or DPPC but the increase was more pronounced for the apo A-IV/DMPC particles. In both complexes, the alpha helical amphipathic segments of the protein were parallel to the lipid acyl chains and no significant modification of the overall organization of the lipid molecules in the lipid bilayer was observed. The presence of apo A-IV seems only to affect the conformation of the lipid hydrocarbon chains in close contact with the protein in the discoidal particles.