Direct measurements by fluorescence correlation spectroscopy of lateral diffusion coefficients of fluorescent lipid analogs in lipid bilaryer membranes indicate self-diffusion coefficients D greater than 10(-7) square centimeters per second for various lipid systems above their reported transition temperatures. Cholesterol in egg lecithin at mole ratio of 1 : 2 reduces D by about twofold, while retained hydrocarbon solvent can increase it by two- to threefold.

The lateral-diffusion coefficients (D) of a fluorescent lipid analogue, 3,3'-dioctadecylindotricarbocyanine (diI), have been measured in black lipid membranes (BLMs), in large (20--50-micron diameter) solvent-free bilayer and multilayer membrane vesicles, and in multilamellar liquid crystals of dipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine, and egg lecithin. They show that D changes by several orders of magnitude at the liquid-crystal transformations of the solvent-free bilayers and multilayer. In all BLMs, D approximately 10(-7)cm2/s with only weak temperature dependence even near the putative phase-transformation temperatures, Tt. In the corresponding liquid crystals and large vesicles, D approximately 10(-8)cm2/s above Tt, decreasing by about two orders of magnitude to D approximately 10(-10)cm2/s below Tt. The changes of D in bilayer vesicles suggest that the expected liquid-crystal phase transitions from smectic A to a higher ordered state do persist in isolated bilayers. Retained solvent in black lipid membranes formed by both Mueller--Rudin and Montal--Mueller methods appears to enhance lateral diffusion. A simple method of forming small numbers of large solvent-free vesicles is described.