In a recent classification of biologically active amphipathic alpha-helixes, the lipid-associating domains in exchangeable plasma apolipoproteins have been classified as class A amphipathic helixes (Segrest, J.P., De Loof, H., Dohlman, J.G., Brouillette, C.G., Anantharamaiah, G.M. Proteins 8:103-117, 1990). A model peptide analog with the sequence, Asp Trp Leu Lys Ala Phe Tyr Asp Lys Val Ala Glu Lys Leu Lys Glu Ala Phe (18A), possesses the characteristics of a class A amphipathic helix. The addition of an acetyl group at the alpha-amino terminus and an amide at the alpha-carboxyl terminus, to obtain Ac-18A-NH2, produces large increases in helicity for the peptide both in solution and when associated with lipid (for 18A vs Ac-18A-NH2, from 6 to 38% helix in buffer and from 49 to 92% helix when bound to dimyristoyl phosphatidylcholine in discoidal complexes). Blocking of the end-groups of 18A stabilizes the alpha-helix in the presence of lipid by approximately 1.3 kcal/mol. There is also an increase in the self-association of the blocked peptide in aqueous solution. The free energy of binding to the PC-water interface is increased only by about 3% (from -8.0 kcal/mol for 18A to -8.3 kcal/mol for Ac-18A-NH2). The Ac-18A-NH2 has a much greater potency in raising the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine than does 18A. In this regard Ac-18A-NH2 more closely resembles the behavior of the apolipoprotein A-I, which is the major protein component of high-density lipoprotein and a potent inhibitor of lipid hexagonal phase formation. The activation of the plasma enzyme lecithin: cholesterol acyltransferase by the Ac-18A-NH2 peptide is greater than the 18A analog and comparable to that observed with the apo A-I. In the case of Ac-18A-NH2, the higher activating potency may be due, at least in part, to the ability of the peptide to micellize egg PC vesicles.