In order to clarify the significane of the spleen in relation to hematologic disturbances in Banti's syndrome, comparative studies between splenic arterial and venous blood elements and histologic examination of the spleen were carried out in fifteen cases of Banti's syndrome and ten control cases. The following results were obtained. 1) In cases of Banti's syndrome, red cell count, hemoglobin content, hem-atocrit value, red cell fragility, average diameter of red cells, white cell count, differential white cell count, platelet count, plasma protein value, erythrocyte sedimentation rate and transaminase activity showed little difference from those of the control cases except for less pronounced tendency toward increase of white cell count in splenic venous blood. 2) The difference in oxygen contents between splenic arterial and venous blood in Banti's syndrome was greater than those of the control cases. This difference was more evident in type III than in type II of the Banti spleen. 3) Histologic examination of the spleen of Banti's syndrome and the control cases revealed that phagocytosis or fragmentation of blood cells were hardly noticeable in both groups. 4) Concerning the erythrocyte distribution in the splenic tissue, a larger number of red cells were seen in the pulp cords in cases of Banti's syndrome than in those of the control. This was particularly so in the spleen of type III as compared with type II.