The physicochemical and mechanical properties of phospholipids dispersed in an aqueous medium provide the molecular framework for many of the dynamic properties of cell membranes (1 ,2 ). The ability of protein molecules embedded in the plasma membrane to move laterally from one location to another, for example, in lymphocyte capping, depends on the presence of a low-viscosity (fluid) lipid domain in the membrane. Traffic of membrane materials to and from the cell surface depends on the ability of membrane vesicles to fuse with and to bud from the plasma membrane. Such processes are essential for the release of neurotransmitters into the synaptic cleft and play a role in a variety of diseases involving infection of cells by enveloped viruses, such as influenza virus (3 ).
