The properties of phosphatidylcholine/cholesterol vesicles have been studied extensively because of their relevance to the behavior of these components in cell membranes. At equilibrium, phosphatidylcholine bilayers are saturated when equimolar levels of cholesterol are incorporated; the cholesterol molecules interfere with the cooperative lateral interactions of the phosphatidylcholine acyl chains and restrict the fluidity relative to pure liquid-crystal phosphatidylcholine bilayers. Mixed cholesterol/phosphatidylcholine bilayers containing more than equimolar cholesterol are metastable; on storage excess cholesterol is released from the vesicles and forms cholesterol monohydrate crystals. This process models the formation of cholesterol gallstones in bile and the growth of the crystals probably involves, at least in part, diffusion of cholesterol molecules from the vesicle bilayer to the crystal surface. The cholesterol-phosphatidylcholine interaction energy in the lipid-water interface of the donor vesicle has a critical effect on the rate of this transfer process.