After treatment of blood smears from human, chicken, pigeon, pig, mouse, rabbit, lamb and horse with aqueous or hydroalcoholic solutions of morin, cytoplasmic granules in heterophil and eosinophil leukocytes fluoresce in a bright yellow colour under violet-blue exciting light. Alkaline solutions of morin show an absorption maximum at 390 nm. Under excitation at 440 nm, peaks of fluorescence emission at 505, 530, 550 or 570 nm are found, in dependence on the concentration, solvent and pH. Extraction and blocking procedures applied on pig and chicken blood smears suggest that stacking interactions of morin with basic components and perhaps with metal cations could be implicated in the specific fluorescence of acidophilic granules. The morin fluorescence reaction permits the precise visualization of eosinophil and heterophil granules in a rapid way and could be useful to identify different cell lines under pathological conditions and in comparative hematologic studies.