Apolipoprotein A-II (apoA-II) is a dimeric 77-residue apoprotein of human high-density lipoproteins. Previous studies indicate that residues 56--77 in the apoprotein do not bind phospholipid whereas residues 47--77 form a complex with dimyristoylphosphatidylcholine (DMPC). To further delineate the lipid-binding region between residues 47 and 77, we have prepared synthetic fragments of apoA-II corresponding to residues 54--77, 52--77, and 50--77 and have tested each fragment for its ability to interact with vesicles of DMPC. The interaction of the fragments was determined by changes in secondary structure as measured by circular dichroism and by isolation of peptide--DMPC complexes by ultracentrifugation in density gradients of KBr. By these criteria, only fragment 50--77 binds DMPC; there is an increase in alpha helicity from 17% to 41% when the fragment associates with lipid. Since the 56--77 fragment does not associate with phospholipid, we propose that the addition of residues Thr-Pro-Leu-Ile-Lys-Lys (corresponding to residues 50--55) to the 56--77 fragment gives the peptide the necessary sequence information for lipid binding. To further identify the important amino acid residues in the region of 50--55, we have substituted Leu-Ile with Ala-Ala. This substitution totally abolishes the lipid-binding capacity of the 50--77 fragment. On the other hand, substitution of Lys-Lys with Ser-Ser does not alter the lipid-binding capacity of the peptide. We conclude that residues 50--55 are important in lipid binding and that the hydrophobic center formed by Leu-Ile plays an important role.