The amphoteric acid dyes acid fuchsin, aniline blue WS, azocarmine G, fast green FCF, light green SF and the wholly acid dye thiazine red R were used for fluorochromy of mast cells in sections of formalin or alcohol-formalin fixed rat tongues. By ordinary light the mast cell granules were barely visible. By UV or blue light exitation, however, a selective, secondary fluorescence of varying intensity and contrast against the tissue background manifested itself. By thin layer chromatography each dye revealed 3-7 components. Except for aniline blue WS, the main component(s) of each dye conferred to the granules the same fluorescence as the corresponding whole dye. These results of fluorochromy, together with the findings of methylation and deamination experiments, allowed the assumption that the anionic groups of the dyes were bound to unoccupied basic groups of the mast cell. granules.