It was previously reported by the author that an abnormal severe transition of iron has been recognized in the body of a horse infected with equine infectious anemia (ETA). In this horse, a large number of tissue cells of the spleen were destroyed severely after in fection with ETA. Then, the storage iron was spilt from the destroyed tissue cells into the circulating blood. In the case of ETA, most siderocytes were produced from the cells which had been originated from tissue and blood and which had absorbed ferritin and hemosiderin as storage iron. Consequently, it was clearly understood what was the mechanism of for mation of siderocytes which had been detected in the peripheral blood of a horse infected with ETA. If the above-mentioned results are always obtainable, a difference will be found in the iron figure of the siderocyte between the acute case of ETA which has severely destroyed tissue cells and in which a large quantity of iron has been spilt into the blood stream, and the chronic case which has only slightly destroyed tissue cells and in which a small quantity of iron has been spilt in the blood stream. Therefore, comparative investigation was carried out on the iron figure of the siderocyte between the acute and chronic case of ETA. The siderocytes were morphologically divided into the following three types by iron figure : (a) FG type which had granules stained blue owing to the presence of hemosiderin and contained in the cytoplasm, (b) FD type which had cytoplasm diffusely stained blue owing to the presence of ferritin, and (c) FGD type which had cytoplasm diffusely stained blue and containing granules stained blue owing to the presence of ferritin and hemosiderin. When investigation was made in an acute case and two chronic cases of ETA, the FG type was abundantly found in the acute case andthe FD type in the chronic cases. In short, it was recognized that there was a difference in the iron figure of the siderocyte between cases produced by different pathogenesis of ETA.