Aim: To localise the recently discovered glycoprotein opticin in the adult human eye. Methods: Polyclonal rabbit antisera were raised against two different opticin peptides. Isolated human vitreous collagen fibrils were extracted with 8 M urea and the extract analysed by SDS-PAGE and western blotting. Paraffin embedded sections from two normal eyes were subjected to immunohistochemical analysis. Results: Western blot analysis of the vitreous collagen fibril extract specifically identified opticin as a 45-50 kDa component that migrated as a doublet. Opticin was especially immunolocalised to the vitreous humour where labelling was most intense in the basal and cortical vitreous gel and less intense in the central vitreous. In addition, specific staining was observed along the surfaces of adjacent basement membranes including the internal limiting membrane (ILM) and posterior capsule of the lens. In one eye, labelling was also observed on the anterior lens capsule, but no other ocular tissues were specifically labelled. A type XVIII collagen/endostatin antibody labelled several ocular tissues including the ILM and basal vitreous gel. Conclusion: The immunolocalisation of opticin was confined to the vitreous humour, ILM, and lens capsule. In situ hybridisation studies have previously demonstrated opticin expression by the posterior non-pigmented ciliary epithelium. Thus, the immunolocalisation data support the proposition that the non-pigmented ciliary epithelium secretes opticin into the vitreous cavity where it associates with vitreous collagen and adjacent basement membranes. The staining along the ILM suggests a role for opticin in vitreoretinal adhesion and the co-localisation of opticin with type XVIII collagen/endostatin at the ILM raises the possibility that interactions between these two molecules might contribute to vitreoretinal adhesion.