In the investigations of the relationship between the antigen-antibody reaction and eosinophilia by bone-marrow tissue culture (solid medium), the author obtained the following results: 1. When the bone marrow tissue obtained from guinea pig sensitized with cow's serum with addition of corresponding antigen 2 to 3 weeks after sensitization is cultured, marked eosinophilia in the growth zone and a marked acceleration in the wandering velocity of eosinophils have been recongized. In addition, mitotic picture of young eosinophils can be obeserved in the tissue slice specimens prepared with the same bone marrow thus cultured. 2. When cow's serum (serum of different species of animal) is added to the bone-marrow tissue culture of normal guinea pig, eosinophilia and acceleration in motility can be observed as in the case using the plasma of guinea pig sensitized with cow's serum and taken 2-3 weeks after sensitization, although hardly any change can be observable in the case using the plasma of normal guinea pig. 3. In the lung tissue culture of the guinea pig in the state of anaphylaxis by reinjection of antigen, many eosinophils have been recognized to have accelerated motility. From these findings, it is assumed that there is a close relationsip between the antigen-antibody reaction brought about by foreign serum and eosinophilia, and that during this allergic reaction eosinophils of bone marrow multiply themselves by mitosis, and in addition, those eosinophils with a markedly accelerated motility wander out from the medulla into the venous sinuses in a great number, thus causing terminal blood eosinophilia.