Hyaluronic acid was visualized using a new histochemical technique: the hyaluronic acid-binding region of the cartilage proteoglycan was isolated, linked with biotin and used in histological sections as a ligand for hyaluronic acid. Staining was performed using the avidin-biotin-peroxidase technique. The presence of hyaluronic acid in the anterior segments of rabbit and monkey eyes was studied in fresh-frozen, as well as in fixed paraffin sections; addition of cetyl-pyridinium chloride to the fixative was essential. In both species intense staining was seen in the stroma of the conjunctiva, the connective tissue of the ciliary processes, and on the apical membranes of the corneal endothelium. Species differences were observed in both conventional and unconventional outflow pathways. In rabbits, the tissue located directly adjacent to the aqueous plexus and the connective tissue surrounding the poorly developed ciliary muscle were stained. In monkeys, only the anterior, non-filtering part of the trabecular meshwork and the outer wall of Schlemm's canal showed intense staining in paraffin sections. In frozen sections, the inner wall (cribriform region of the trabecular meshwork) was also stained. The spaces between the anterior part of the ciliary muscle bundles remained almost unstained.