Chromatographyically and immunologically homogeneous apolipoprotein A-I (apoLp A-I) from human serum has been recombined in separate experiments with three species of sphingomyelin. The differed in the degree of saturation of their fatty acyl residues, stearoyl (18:0), oleoyl (18:1) and linoleoyl (18:2). The lipoprotein complexes formed were purified by CsCl density gradient centrifugation between 1.07 - 1.09 g/cm3 and by gel filtration. Stearoylsphingomyelin does not recombine with the apoprotein A-I below its phase transition temperature (tc = 41.5 degrees C). The lipoproteins eluted with the following apparent molecular weights: 18:0-sphingomyelin apoLp A-I, 8.0 X 10(5); 18:1-sphingomyelin apoLp A-I, 4.0 X 10(5); and 18:2-sphingomyelin apoLp A-I, 4.0 X 10(5). In electron microscopy the particles appear as discs of 160 - 170 A diameter and 50 - 60 A thickness. Their tendency to form stacked aggregates of discs decreases with the degree of their unsaturation. CD measurements underline the considerable increase in alpha-helicity of the secondary structure of apo A-I after recombination with the phospholipids. This increase in order is equal for the three sphingomyelin species (alpha-helicity of apoLP A-I = 0.46, after recombination 0.89). If the three sphingomyelin species are used in equal molar amounts in the recombination experiment, no preference for any one sphingomyelin species is observed. Recombination of apoLp A-I with sphingomyelin, labelled with the isotope 13C in the choline group, C-14 of stearic or linoleic, or C-11 of oleic acid, were performed for spin lattice relaxation time (T1) experiments. Compared with sphingomyelin liposomes, the polar head groups of these lipids in the lipoprotein particles possess a considerably higher mobility, whereas the changes in T-1-times of the C-atoms in the centre of the fatty acid chains of the lipids refer to their interactions with the polypeptide side chains. A model of the lipoprotein complexes formed is proposed on the basis of the experimental data.