Superoxide dismutase and catalase activities have been measured in cell-free extracts of calf trabecular meshwork, and for comparison, in calf iris, retina, lens, liver, and erythrocytes. Gel electrophoresis has been used to identify isozymes of each enzyme. The superoxide dismutase and catalase activities per milligram wet weight of calf trabecular meshwork, 0.184 and 0.884 U/mg wet wt, respectively, were comparable to those found in iris and retina, and much higher than those found in lens. Three isozymes of superoxide dismutase were identified in trabecular meshwork. Two of these presumably correspond to cytoplasmic superoxide dismutase, while the third corresponds to a mitochondrial isozyme. A presumably mitochondrial superoxide dismutase activity was also observed in iris and retina, but not in lens. A single catalase isozyme was found in all tissues examined. At physiologic H2O2 concentrations, catalase may have similar levels of activity to glutathione peroxidase. Superoxide dismutase, catalase, and glutathione peroxidase may constitute an important defense mechanism of trabecular meshwork against the toxic O2-. and H2O2 to which it must be continuously exposed from endogenous production as well as from the aqueous humor.