Lipid bilayer membranes are used to study various aspects of membrane structure and function. Electrical properties of bimolecular lipid membranes (BLM's) are measured to study: (1) mechanisms of opening and closing ion channels; (2) properties of ionophores isolated from natural cell membranes; (3) mechanisms of immune cytotoxicity; (4) relationship between ion permeability and mitogenesis. Diffusion and patching of fluorescent surface receptors in BLM's is studied by the technique of fluorescence photobleaching recovery: a small spot on the fluorescent-labeled BLM surface is photolytically bleached by brief exposure to an intense laser beam, and subsequent recovery of the fluorescence is monitored. Lipid bilayer vesicles containing a high concentration of the fluorescent dye 6-carboxyfluorescein (6-CF) are used to study: (1) mechanisms of liposome-cell interaction (e.g., surface adsorption, fusion or endocytosis), and (2) the mechanism of lysis of the vesicles by drugs or by complement. The vesicles are incubated with human peripheral blood lymphocytes, and the resulting fluorescence is observed by microscopy, by fluorometry and by use of the fluorescence-activated cell sorter. BIBLIOGRAPHIC REFERENCES: Weinstein, J. N., Yoshikami, S., Henkart, P., Blumenthal, R. and Hagins, W. A.: Liposome-cell interaction: Transfer and intracellular release of a trapped fluorescent marker. Science 195: 469-492, 1977. Rosenstreich, D. L. and Blumenthal, R.: Ionophorous activity and Murine B-lymphocyte mitogens. J. Immunol. 118: 129-136, 1977.