The phenomenon of autofluorescence is reported in the thymus of rabbits; besides reticular epithelial cells, Hassall's bodies also emit autofluorescence. The staining of thymic corpuscle by fluorescein isothiocyanate is also described. This staining is important not only because FITC is commonly used in immunofluorescence and may lead to misinterpretations due to false-positive results, but also for it permits a good precision in the observation of the relationship between Hassall's bodies and reticular epithelial cells. Some hypothesis are set up to explain the autofluorescence, and the fluorochromization of Hassall's bodies by acid fluorochromes.