An assay to study the spontaneous charged lipid transfer between lipid vesicles is described. A donor/
acceptor vesicle system is employed, where neutrally charged acceptor vesicles are uorescently
labelled with the electrostatic membrane probe Fluoresceinphosphatidylethanolamine (FPE).
Upon addition of charged donor vesicles, transfer of negatively charged lipid occurs, resulting in
a uorescently detectable change in the membrane potential of the acceptor vesicles. Using this approach we have studied the transfer properties of a range of lipids, varying both the headgroup and the chain length. At the low vesicle concentrations chosen, the transfer follows a rst-order process where lipid monomers are transferred presumably through the aqueous solution phase from donor to acceptor vesicle. The rate of transfer decreases with increasing chain length which is consistent with energy models previously reported for lipid monomer vesicle interactions. Our assay improves on existing methods allowing the study of a range of unmodi ed lipids, continuous monitoring of transfer and simpli ed experimental procedures.