Antisera to extracts of human hyaline cartilage allow distinct visualization of extracellular structures of adventitial sheaths around arterial capillaries in the red pulp and marginal zone in the human spleen. These pericapillary sheaths have a prominently spongy character indicating their essential filter function. In childhood they are most marked. In premature and stillborn infants they sometimes occupy whole stretches of pulp cords, whereas in old age they are frequently diminished to small formations lacking a distinctly spongy structure.