Abstract
Procion Yellow M-4RS, an intracellular neuronal marker, has been shown to bind covalently to the subcellular organelles of rat brain under physiologic conditions. Binding to purified proteins, nucleic acids and lipids also occurs under such conditions, the probable site of attachment being the primary amino group. Lipid membranes were highly impermeable to this dye. The fluorescence intensity of Procion Yellow M-4RS was shown to be viscosity-dependent. These results are consistent with the behavior of the dye when it is used for mapping neurons.