Micromechanical tests of giant vesicle bilayer elasticity and bilayer-bilayer adhesivity have been carried out on vesicles made from mixtures of lipids, cholesterol and polypeptides. Mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) exhibited ideal solution behaviour over a temperature range that covered both liquid-to-gel phase transitions. Addition of cholesterol (CHOL) to a saturated chain lecithin (DMPC) reduced, broadened and shifted to higher temperature the main crystalline-to-liquid acyl chain transition. Cholesterol greatly reduced the membrane area compressibility and increased membrane cohesion to levels exhibited by frozen acyl chain bilayers, but maintained the bilayer in a liquid state. Addition of amphiphilic polypeptides to PC and PC-CHOL mixtures slightly increased bilayer compressibility at high temperatures; when the temperature was lowered, bilayer compressibility for DMPC-CHOL-peptide mixtures was greatly reduced to below that of the single component lipid. Cholesterol appeared to change from an association with the lipid at low temperatures to an association with the protein at high temperatures. Membrane cohesion correlated with a simple fracture energy model where the level of tension required to lyse vesicles is proportional to the square root of the elastic area compressibility modulus. Free-energy potentials for adhesion of mixed lipid (PC and PE) bilayers showed a dramatic increase as the mole fraction of PE approached unity. Based on published values of separation distance between lamellae at full hydration for each pure component, the free energy potentials for adhesion of mixed-lipid bilayers were calculated from theoretical models of the van der Waals attraction and hydration repulsion then compared with the measured values of adhesion energy. The correlation provides strong evidence that the hydration repulsion can be represented by a surface-potential theory.