From studies involving 31 cataracts classified by the CCRG system and eight normal human lenses, it has been found that the adult human lens contains an enzyme system capable of oxidizing 1-2 mumol of glyceraldehyde, acetaldehyde, propionaldehyde, formaldehyde, and malonaldialdehyde per hour to their carboxylic acid form. Roughly 30 mumol G-3-P can be oxidized per hour. Statistically, the level of the oxidase system in nuclear cataracts and deeply pigmented lenses was found to be the same as for normal lenses. The deficiency of an enzyme responsible for the oxidation of highly reactive aldehydes thus seems unlikely to be involved in nuclear cataract formation and the browning of the lens. Evidence that the observed oxidase activity occurs via two separate enzymes: aldehyde dehydrogenase and glyceraldehyde-3-P dehydrogenase was achieved by studying the response of enzyme to substrate and activators (dithiothreitol and arsenate) and by final separation of enzyme activities. Differences in pH optima and heat treatment response further distinguished one enzyme from the other.