Human corneal aldehyde dehydrogenase (designated ALDH3) was purified to homogeneity and characterised with respect to substrate specificity and inhibition by thiol reagents. The enzyme was present as a major soluble protein (5% of the total soluble protein) and was found to efficiently catalyse the oxidation of medium chain peroxidic aldehydes which may be found in the cornea. These findings are consistent with the proposal that ALDH3 plays a dual role in the absorption of UVR and in the oxidation of peroxidic aldehydes in the mammalian cornea. Disulfiram did not inhibit this enzyme under the conditions used in this study, however p-hydroxymercuribenzoate rapidly inactivated the enzyme. Analysis of the proteins of the cornea and surrounding tissue indicated that in both the cow and the human, changes in the nature and quantity of soluble proteins occurred. Phenotype variants of the ALDH3 were apparent in a small Australian population.