By intraperitoneally injecting to normal guinea pigs the antiserum of bone marrow obtained from the normal rabbits to which the intraperitoneal injection of the guinea-pig bone marrow had been previously given, the author studied the changes in the peripheral blood and bone-marrow pictures in these guinea pigs; and arrived at the following conclusions: 1. By the intraperitoneal injection of bone-marrow anti-serum a transient hyperchromic anemia can be recognized in the test guinea pigs. Namely, the number of erythrocytes and the hemoglobin (Hb) content are decreased, and anisocytosis, erythroblasts, polychromatic erythrocytes, and the erythrocytes possessing basophilic spots are recognized. Reticulocytes have been found to have increased in some of the test animals. 2. The number of leucocytes fluctuates with the injection of the anti-serum of bone marrow, and in the relatively greater number of animals leucocytes tend to decrease at first and later increase. In the leucocyte percentage, a decrease in pseudoeosinophils, and an increase in lymphocytes can be observed in most of animals, and also eosinophils increase in number. Besides these, the appearance cf young granulocytes such as myelocytes and metamyelocytes as well as the regressive degeneration of various leucocyte cells can be recognized. 3. Platelets are decreased in number in all cases with the injection of bone-marrow anti-serum. 4. As for the bone-marrow picture in the majority of the test guinea pigs the interferance with the production of erythroblasts can be observed, and in some of the test animals the impairment of maturation of erythroblasts and in another part of the test animals the left shift of nuclei in pseudo-eosinophils can be seen. 5. Judging from the relatively slight effect of the normal rabbit serum on the peripheral blood and bone marrow of guinea pigs, it is assumed that the anemia inducing action of the bone-marrow anti-serum is specific.