Our present studies of the Cyprinoid spleen are primarily confined to only two varieties of carp, namely, Cyprinus carpio L. and Carassius auratus L. The spleen of both fish is very much the same, being of a single thin layer of epithelium with a thin stratum of fibrous tissues underlying it. The capsule is devoid of muscles and does not have the dense trabeculae extented into its tissues as in the mammals. The chief characteristics of the spleen are the ellipsoids, pulp and melanomacrophage centres.Ellipsoids: At each of the terminal end of the splenic arteries, it branches into 3 to 4 thick-walled filter capillaries, designated as "ellipsoids" similar to the branching of the brush-artery (a. penicilli) in mammals. The lumen of the ellipsoids is rather narrow. They have a flattened endothelial lining and external to which is a single layer of large macrophages. This macrophage sheath is bounded by a fairly thick fibrous membrane, its lumen finally opens into the pulp lacunae. In the lacunae of the spleen, newly formed blood cells enter into the blood stream. From the lacunae the blood passes into the venous capillaries and venules, and in turn drains into the veins adjacent to the arteries. Since the lumens of the ellipsoids are very narrow and blood cells must pass through them, the degenerating blood cells (especially the erythrocytes) would likely be damaged during their passage and eventually disintegrated. Their hemoglobin, especially the iro nmolecules, are utilized in the formation of new blood cells.The spleen of the Crucian Carp was also examined for their phagocytic properties with colloidal carbon by intraperitoneal injection. Fish are killed at 2,5,24 and 48 intervals h At 2 h. after injection only a small number of macrophages laden with carbon particles were seen within the ellipsoids. At 48 h. there were many macrophages densely packed with carbon particles in the ellipsoids. Electron micrographs showed that in addition to the fine carbon particles there are also numerous round residual bodies observed in the macrophages of the ellipsoids.Splenic pulp: There is no sharp demarcation between white and red pulp; the splenic pulp consists of many blood lacunae without vascular walls. In the lacunae of the spleen there are a large number of red blood cells, lymphocytes and hemopoietic tissues. The newly formed blood cells enter into the blood stream, and passing through the venous capillaries and venules. Imprints of the spleen showed that mature erythrocyte, lymphocytes, thrombocytes and hemocytoblasts are the commonest cellular elements. The granulocytes are also seen though not prominent.Melanomacrophage centres: The melanom acurophage centres are usually located nearavessel, and may often be invested with one or more layers of strands of externa, usually a fine reticular limitting membrane. Autoradiographic studies by Ellis and de Sousa (1974) concluded that replete macrophage may migrate from the ellipsoids to the melanomacrophage centres and suggested that the aggregates of macrophages and melanomacropbages with lymphoid cells in the spleen of the teleost fish might represent primitive germinal centres. According to our observation, the central region of the germinal centres of the lymphoid tissue is filled with immature lymphocytes exhibiting many mitotic figures. When the Crucian Carp are injected with indian ink, the number of melanomacrophage centres increased and are more distinct. It is capsulated with 3 to 4 concentric rings of flattened fibroblast like cells with indistinct central cells, their nuclei collapsed, desolveed and eventually completly disintegrated. In view ow these facts, we incline to believe that the function of the fibrous and the flattened cell capsules of the melanomacrophge centres might also act as a factor to prevent the autolytic products to leak out.