Splenic Schweigger-Seidel sheath or ellipsoid was studied by anti-vimentin monoclonal antibodies, clones 3B4 and V9. The 3B4 mAb recognizes a cell type with cytoplasmic granules and long processes which make a network around the endothelial cells of the penicilliform capillaries. The V9 mAb identifies the reticular cells of the spleen which form the supporting system of the Schweigger-Seidel sheath. The sheath covers the entire length of the penicilliform capillary from the central artery, including the branching area. The appearance of the Schweigger-Seidel sheath in a section depends on the section plane; namely, at the branching area its shape is highly irregular, while close to the red pulp it looks like a sleeve. Therefore, the English term "ellipsoid" does not cover the morphological appearance of this unique splenic structure; the Schweigger-Seidel sheath is a correct term.