In human cornea an anion sensitive ATPase is present. The highest specific activity was found in the endothelium, whereas the epithelium contained the highest total activity. The enzyme was stimulated by bicarbonate and sulfite and inhibited by thiocyanate. The majority of the enzyme was localized in the mitochondria but some activity was also detected in the plasma membranes. Atractyloside inhibited only the mitochondrial anion ATPase.