The cell-impermeant, fixable, polar tracer Alexa Fluor™ 594 biocytin combines the red-fluorescent Alexa Fluor™ 594 fluorophore with biotin and an aldehyde-fixable primary amine. Polar tracers are commonly used to investigate cell-cell and cell-liposome fusion as well as membrane permeability and transport through gap junctions or cell uptake during pinocytosis. This water-soluble tracer can be introduced into cells by whole-cell patch clamping, iontophoresis, osmotic lysis of pinocytic vesicles or comparable methods. The biotin allows for detection and subsequent amplification with enzyme-modified streptavidins including horseradish (HRP) and alkaline phosphatase (AP) that can be used with TSA and ELF™ technologies, respectively.