Abstract –A previously described method in which mature rat mast cells stained with acid dyes were demonstrated by their selective fluorescence, was further studied. The material consisted of tongues from rats and mice in addition to heads and tails from 1‐d‐old rats and embryos 19 d post gestation stage. Chronically inflamed human gingiva obtained from gingivectomies was also studied. Some basic dyes were used for control and comparison. With the acid dyes acid fuchsin, fast green FCF and azocarmine G a mast cell fluorescence equal to that in rats was also demonstrated in mice and humans. Immature rat mast cells stained with acid dyes disclosed a selective fluorescence, the intensity of which was, however, lower than that observed with the basic dye acridine orange. Immature mast cells in the chronically inflamed gingiva revealed a fluorescence of lower intensity than that emitted by the mature cells. In mature mast cells from rats and mice the fluorescence color induced with acid dyes except for thiazine red R, was dependent on the dye concentration. No or insignificant change of fluorescence color was observed with the basic