Summary: NC-strain mice have a constitution, that anaphylactic shock is very likely to occur, so if ovalbumin is administered in terms, almost all the mice die of shock by the third injection. The more sensitizations were performed, using ovalbumine as an antigen, the profunder became the mices allergic condition. The number of mast cells increased especially in the fascicular zone of the adrenal glands and the fluorescence intensity of them became stronger. Under light microscopic studies, the granules in a cell decreased and they were stained bluish purple by toluidine blue and under electron microscopic studies the number of the granules increased with high electron density. Once, either ovalbumin or compound 48/80 was injected under these allergic conditions, the mice showed remarkable symptoms of allergy and many of them died of anaphylactic shock. The fluorescence of histamine in the mast cells decreased and most of the mast cell granules stained pink. Under the electron microscopic studies the density of the granules decreased extremely and most of them were swollen, some of them united with each other and others released themselves from the plasma membrane. Whenever prednisolone was injected the day before the challenge or compound 48/80 the allergy symptoms were not serious. The granules became small and stained dark blue and there occurred only a small amount of degranulation. The density of the granules increased tremendous and there were many granules which had a surrounding space between the cytoplasm and themselves. Through pretreatment with prednisolone, the fluorescence intensity of histamine became slightly weaker than before. But after the challenge or compound 48/80, it scarcely changed. The symptoms of shock, changes of granules and the release of histamine etc. were clearly protected by pretreatment with prednisolone.