Opticin is a class III member of the extracellular matrix small leucine-rich repeat protein (SLRP) family that was initially identified in the eye in association with the collagen fibrils of the vitreous humor. Recombinant and tissue-extracted forms of bovine opticin were subjected to biochemical and biophysical characterization. Following SDS-PAGE the predominant component produced by both forms was a broad band between 45-52 kDa. There was evidence for two-stage processing and, additionally, a proteolytic cleavage product of ∼25 kDa. Deconvolution of circular dichroism spectra revealed β-sheet (41%), β-turn (21%), and α-helix (10%), and thermal denaturation experiments showed a transition with a midpoint of 47°C. Weight-averaged molecular mass measurements using both light scattering and analytical ultracentrifugation demonstrated that opticin exists in solution as a stable dimer of ∼90 kDa, which can be dissociated into a monomer by denaturation with 2.5 M guanidine hydrochloride or during SDS-polyacrylamide electrophoresis. Opticin remains a dimer after removal of the amino-terminal region by O-sialoglycoprotein endopeptidase digestion, suggesting that dimer formation is mediated by the leucine-rich repeats. Dimerization could have a number of functional consequences, including divalent ligand interactions.