There is no dispute that alcohols alter lipid bilayer properties, as assayed with a variety of methods, but are these effects sufficient to alter membrane protein function at the concentrations where they alter biological functions? Do direct alcohol-protein interactions dominate, or are the biological effects of alcohols due to a combination of direct and bilayer-mediated effects? To address these questions, and to explore the general structure-activity relationship for amphiphiles, we determined the bilayer-modifying potency of various alcohols using a gramicidin (gA)-based fluorescence assay (37,38). gA channels are formed by the transbilayer dimerization to two nonconducting subunits (39). The channel's hydrophobic length is usually less than the bilayer's hydrophobic thickness, meaning that the bilayer adapts locally to accommodate the channel. This adaptation, or deformation, has an associated energetic cost such that the gA channel monomer↔dimer equilibrium is energetically coupled to the cost of deforming the bilayer (40–43). gA channels thus serve as molecular force probes to assay changes in lipid bilayer properties, as sensed by a bilayer-spanning channel (42,45). Changes in bilayer properties that alter the energetics of the gA monomer↔dimer equilibrium alter the time-averaged number of conducting gA channels, which can be assayed from the changes in the rate of fluorescence quenching.