Mammalian eosinophil leucocytes contain specific lysosome- and peroxisome-like cytoplasmic granules that have important implications in inflammation, allergy and the bactericidal response to  microorganisms. Several highly cationic proteins are responsible for the intense acidophilia of eosinophil granules, which also have a characteristic ultrastructure consisting of a dense core and an external matrix. The granules of horse eosinophils are large in size and are easily recognizable as individual elements by light microscopy. These characteristics make these cells an adequate model for cytochemical analyses and precise light-electron microscopical correlations. In this work, the selective and cytochemical staining and fluorescence reactions applied to horse blood smears are reviewed, and show that eosinophil granules are very suitable structures for testing new light microscopical methods. Transmission electron microscopy has shown that horse eosinophil granules have an electron-dense, non-crystalline core surrounded by a less dense external matrix, although there is considerable heterogeneity in their ultrastructural morphology. Cytochemical results show a ring-like pattern for some staining and fluorescence reactions (glutaraldehyde-oxidized hematoxylin, fast green FCF, 1-hydroxy-3,6,8-pyrene-trisulfonate at pH 12, Timm sulfide-silver method), indicating that the external matrix of horse eosinophil granules contains metal cations and cationic proteins with high isoelectric points.