Lucifer dyes are intensity fluorescent 4-aminonaphthalimides which are readily visible in living cells at concentrations and levels of illumination at which they are nontoxic. Because of their low molecular weight they frequently pass from one cell to another; this widespread phenomenon, termed dye-coupling, is thought to reveal functional relationships between cells. Lucifer dyes can also be used for ultrastructural tracing by comparison of electron micrographs with light micrographs of the same thin section. In addition, they show promise for backfilling neurones through cut nerves, for visualizing the results of retrograde axonal transport and for the covalent labeling of macromolecules.