The frequent occurrence of beta-sheet promoting residues such as Ile, Val, and Thr in the alpha-helical transmembrane segments of most integral membrane proteins suggests that the helical propensities of these residues are altered in the hydrophobic environment of the lipid bilayer. Systematic studies of model peptides by circular dichroism models spectroscopy in various micellar/vesicular media allow the establishment of a ranking order of helical propensity for uncharged amino acids in the membrane environment. In contrast to their conformational preferences in water, the helical proclivity of amino acids in membranes is shown to be governed by their side chain hydrophobicity, and by the hydropathy of the local peptide segments in which the residues reside [corrected].