Localization of catalase in rat and bovine ocular tissues was investigated by an immunohistochemical method. Antisera to purified catalase was raised in a rabbit. Purity of catalase was determined by gel electrophoresis and specificity of the antibody was tested by immunoblot. The immunohistochemical method revealed the presence of catalase predominantly in epithelial and endothelial structures of the eye and in the retina, except in the outer segments. Catalase was found in abundance in those structures that are frequently exposed to oxygen metabolites under physiologic conditions and in such pathologic states as intraocular inflammations. These findings thus suggest that catalase, along with other antioxidant enzymes, may offer protection against the damaging effects of hydrogen peroxide.