We investigated the presence of urokinase-type plasminogen activator (u-PA) in various structures of the human eye by using a biotin-strepavidin complex immunoperoxidase technique with a monoclonal antibody specific for human u-PA. A moderate to intense reaction product was seen in the corneal endothelium, episclera, sclera, iris muscles, lens equator and posterior capsule, peripheral vitreous, epithelium of pars plana, retinal pigment epithelium, axons, uveal fibroblasts, optic nerve fibers, and extraocular muscles. A weak reaction product was seen in the conjunctival and corneal epithelium, the trabecular meshwork, melanocytes and stroma of the iris and choroid, and the posterior layers of the retina. u-PA was not localized in the corneal stroma, conjunctival goblet cells, and certain parts of the lens. Comparison of the present results with those of our previous study (Tripathi, Geanon and Tripathi, 1987, Ophthalmology, 94, 1434-8) on the localization of tissue plasminogen activator (t-PA) in the eye indicate that the distribution of u-PA and t-PA overlaps in many tissues, but differs significantly in others, and that this finding may be related to their complementary and specific roles. As in other body systems, u-PA in ocular tissues is probably involved in processes such as fibrinolysis, wound healing, turnover of intra- and extracellular macromolecules, muscles and neuromuscular regeneration, synapse formation, and maintenance of general neuronal function.