The author has made the electron microscopic study of enzymes in eosinophils in order to clearify the influence of hyposensitization in allergic rhinitis to the activity of enzymes in eosinophilic granules, and the following results were obtained. 1) In all 3 control, hyposensitization and allergic groups, eosinophilic granules with matrix and crystalloid core in circulating blood and tissue were observed. 2) In all 3 groups, activity of acid phosphatase was not found in eosinophil but was found in neutrophil, basophil, macrophage and glands as a form of coagulated activating colony of acid phosphatase. 3) In control and hyposensitizing groups, number of eosinophils were smaller than that was counted in allergic group. Activity of peroxidase in granule was weak, and granular out-flowing and rupture of cell membrane were not observed. 4) In allergic group, eosinophil count was high, activity of peroxidase in granule was strong, and granular out-flowing and rupture of cell membrane were severe. At the same time, many vacuoles, which is suspected to be the result of phagocyte the protein as foreign substance, was observed. Judging from the fact that eosinophil has a specific relation to allergic diseases, and the activity of peroxidase that exist as an enzyme in eosinophilic granule is strong, it is believed that the major function of eosinophil is phagocytosis of antigen, or antigen-antibody complex. On the other hand, the fact that activity of peroxidase was weak in hyposensitizing group lead us to believe that the activity of peroxidase may be used as an indicator for detecting hyposensitizing status in the treatment of allergic disease.