After giving blood depletion, phenylhydrazine, hydrochloric acid hydroxylamine, carbon tetrachloride, and carbon black to rabbits the author estimated the number of Heinz bodies in the circulating blood of each animal and the number of Heinz bodies appearing in the in vitro accelerating test, and studied the relationship between the two appearances. 1. In the circulating blood of normal rabbits no Heinz body can be recognized. By the in vitro accelerating method erythrocytes containing Heinz bodies amount ot 66.7 per cent. 2. In the circulating blood of the rabbits depleted of blood once in a small or a large amount, or successively depleted of blood in an intermediate amount no Heinz body can be found. the circulating blood of the rabbits depleted of its blood once in a large amount a few Heinz bodies can be detected. By the in vitro method Heinz bodies appear in an inverse proportion to the number of erythrocytes, but there seems to be no relation with the number of Heinz bodies in the circulating blood. 3. When phenylhydrazine is injected, Heinz bodies appear in the circulating blood in various forms according to the dose of the injection. However, by the in vitro method the greater majority of Heinz bodies appeared in the circulating blood are destroyed, and many Heinz bodies observable in this instance are the ones produced anew in vitro. 4. In the case injected with hydrochloric acid hydrocylamine, Heinz bodies appearing in the circulating blood are more distinct and erythrocytes themselves are more brilliant than those in the case injected with phenylhydrazine. Even by the in vitro accelerating method Heinz bodies appeared in the circulating biood are destroyed in a lesser number than in the case of the phenylhydrazine injection. 5. In the rabbits administered with carbon tetrachloride Heinz bodies can be recognized in the circualting blood soon after the injection but they are a few in number and small in size. By the in vitro method the blood taken from 10 to 12 days after the injection an intermediate number of Heinz bodies are formed. 6. In the case injected repeatedly with carbon black Heinz bodies appear in the circulating blood 2 to 4 days after the initiation of the injection, and by the in vitro accelerating method Heinz bodies increase as long as the injection is kept up, but they decrease in number at the cessation of the injection. It seems that the function of the reticuloendothelial system is responsible for the formation of Heinz bodies in the circulating blood while the carbon black injection itself is greatly responsible for the appearance of Heinz bodies by the in vitro method. From these, the Heinz body in the circulating blood has the characteristic traits that are different from those of the Heinz body appearing in the in vitro accelerating method. Therefore even the latter is present in vivo in a preparatory state, it is believed that the latter will not immediately be transformed into the in vivo Heinz body.