Two anionic fluorescein derivatives can be used for the simple and definitive localization of neuronal degeneration in brain tissue sections. Initial work on the first generation fluorochrome, Fluoro-Jade, demonstrated the utility of this compound for the detection of neuronal degeneration induced by a variety of well-characterized neurotoxicants, including kainic acid, 3-nitropropionic acid, isoniazid, ibogaine, domoic acid, and dizocilpine maleate (MK-801). After validation, the tracer was used to reveal previously unreported sites of neuronal degeneration associated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), methamphetamine, and d-fenfluramine. Preliminary findings with a second generation fluorescein derivative, Fluoro-Jade B, suggest that this tracer results in staining of optimal contrast and resolution in animals dosed with kainic acid. These 2 tracers can be combined with other histologic methods, including immunofluoresence and fluorescent Nissl stains. Recent preliminary findings on a number of specialized applications of Fluoro-Jade include the detection of apoptosis, amyloid plaques, astrocytes, and dead cells in tissue culture.