Tissue blocks and vascular casts of the human spleen were studied by scanning electron microscopy. Splenic pulp was composed of white pulp packed with lymphocytes and red pulp which consisted of three dimensional reticular meshwork and splenic sinus. There were at least three types of microcirculation with different functions in the human spleen. First, open circulation in the red pulp is engaged in highly sensitive clearance of foreign materials in blood by numerous macrophages resided in splenic cord (culling function). It is also engaged in pitting function by narrow slits of the sinus. The second is the closed circulation occurred in the red pulp in the human spleen. It was represented as an arteriolar labyrinth whose endothelial cells connected with those of sinuses. Blood flow through this circulation is exposed to small number of macrophages in the sinus. It probably corresponds to the fast blood flow in the spleen. Third, microcirculation to the white pulp and marginal zone is engaged in phagocytosis of foreign materials in blood and carriage of their antigenic portions to lymphocytes in the white pulp. Therefore variation of the blood flow through their circulation may bring about a change of the splenic function. Relationship among blood flow, nerve stimulation and immune reaction was discussed.