The water-soluble neuronal tracer 5-(and-6)-tetramethylrhodamine biocytin (biocytin TMR) can be detected in fixed tissues by using avidin or streptavidin probes. However, tetramethylrhodamine biocytin can also be observed directly in fixed or unfixed samples because the attached tetramethylrhodamine dye produces bright, orange-fluorescence (absorption/emission maxima ∼554/581 nm).