The work currently being undertaken places a heavy emphasis on the structural packing of membrane lipids, including cholesterol, and the influence on that packing of membrane molecular probes. These probes include spin-label (both steroid and fatty acid nitroxide free radicals) and fluorescent (anthroyl fatty acid and panmaric acid) molecules. Typical experiments have been carried out using pure and mixed monomolecular forms in a film balance and dipalmitoyl lecithin as a host lipid. Additionally fluorescence depolarization and differential scanning calorimetry experiments have been carried out with liposomes. The results so far indicate that while space requirements are less demanding when an oxazolidine ring or an anthroyl group is near the lipid-water interface, nevertheless such locations affect the behavior of the host lipid to a greater degree than those at or close to the center of the bilayer.