Biocytin (B-1592) is a proven performer, both for neuronal tracing and for following fluid-phase pinocytosis. Like biocytin, our water-soluble Alexa Fluor 546 biocytin can be detected in fixed tissues by using avidin or streptavidin probes. However, Alexa Fluor 546 biocytin can also be observed directly in fixed or unfixed samples because the attached Alexa Fluor dye produces bright, photostable, orange fluorescence (absorption/emission maxima ~553/569 nm).