The lipophilic probe, FM™ 4-64FX exhibits low fluorescence in water but fluoresces intensely upon binding the outer leaf of the plasma membrane providing discrete plasma membrane staining. The binding is rapid and reversible and 'FX' denotes that this analog is fixable with aldehyde-based fixatives. It is also an excellent reagent for identifying actively firing neurons and for investigating the mechanisms of activity-dependent vesicle cycling.