In order to improve our understanding of how lipid organization determines membrane function, experiments are described to elucidate mechanisms of lipid-lipid and lipid-protein interactions. Lipid-lipid interactions between synthetic phospholipid analogs and sterols in vesicles will be examined using permeability and spectral methods. Enzyme activities of lipid-requiring enzymes will be studied in membranes containing these analogs to assess lipid-protein interactions. The consequences of lipid abnormalities that have been described in diseases such as atherosclerosis and red blood cell disorders will be analyzed in terms of membrane organization and intermolecular associations.