Hematological researches in peripheral blood, especially the changes of eosinophils were made from various points in order to solve the nature and etiology of alveolar pyorrhea. The results obtained are summarized as follows: 1) Little difference was recognized on red cell count, hemoglobin quantity and index in comparison with healthy control cases. No remarkable quantitative and qualitative changes of white cells except eosinophils were observed. In addition, any apparent difference between peripheral blood taken from the ear lobe and local gingiva could not be found in these above-mentioned points, 2) Increase of eosinophils was recognized in about 45 per cent of 78 cases, and, moreover, its remarkable relationship to clinical type, especially that of inflammatory findings characterstic of the disease, was suggested. Many cases showing morphological changes, especially degenerative changes of the cells, were observed, and, furthermore, relationlhip between changes of the cells and those of the disease conld be suggested as the experimental results 3) Peroxidizing reaction of the peripheral blood cells by the vitamin C method was investigated in 20 cases showing an increase of eosinophils. Consequently, quite difference could be observed in staining ability of granules between clinical cases and control cases, and thus it could be noticed that granules within the cells in case of alveolar pyorrhea were quantitatively different from those in normal ones. 4) As a result of vital observations on movement of eosinophils by the peripheral blood culture method in 10 cases of the disease, it was recognized that the migration velocity of the cells showed a rapid decrease as time went by and was markedly inferior to that of normal cases and that in the mobility form the cells suggestive findings of declining mobility increased much more than those of normal cases, especially showing a increasing tendency with the progress of culture. All of these various findings of the cells can not necessarily be considered to be the phenomena characteristic of this disease, but pathologic findings and characterics of the cells of this disease may suggest both correlation of the cells to chronic inflammatory changes in case of alveolar pyorrhea and possibility of allergic process as an etiologic factor of the disease.