The retrograde tracer true blue is a UV light-excitable, divalent cationic dye that stains the cytoplasm with blue fluorescence. True blue is reported to be a less cytotoxic retrograde tracer than Fluoro-Gold and to be a more efficient retrograde tracer than diamidino yellow. True blue is stable when subjected to immunohistochemical processing and can be used to photo-convert DAB into an insoluble, electron-dense reaction product.