By means of anti-cartilage antisera, on using immunofluorescent technique, special spongy structures were detected in the red pulp of the human spleen, occurring around arterial capillaries and at their terminals. At some sites, these formations were closely attached to the walls of sinus, suggesting a direct connection between the arterial and venous system. Exceptionally they were found to be attached to the wall of the veins in the red pulp. In the majority of instances, anti-cartilage antisera distinctly visualized also fibrillar and membranous adventitial structures around small arterial branches, which were more or less connected with the spongy structures of pulp cords. In several-month-old infants the spongy structures occupied relatively large areas of pulp cords, and exceptionally also the entire segment of the pulp cord, whereas at advanced age, these structures were very small. Yet they invariably contained structures of sheathed capillaries. These spongy arterial terminals seem to represent specifically formed parts of pulp cords, constituting a particularly specialized part of the vascular system of the human spleen.