1. 1. The enzymatic oxidation of both optical isomers of straight-chain α-hydroxy acids from three through eight carbon atoms, α-hydroxyisovaleric, and α-hydroxyisocaproic acids, and of l-β-imidazole lactic acid has been studied. Several enzyme preparations from rat liver and kidney and from hog kidney were employed. 2. 2. The relative oxidation rates of the above compounds in the presence of a lyophilized, dialyzed hog kidney cortex supernatant preparation were determined. 3. 3. Addition of riboflavin monophosphate to a thoroughly dialyzed hog kidney cortex supernatant preparation produced an approximate doubling in the oxygen uptake. 4. 4. Experiments with washed particle and supernatant preparations from kidney and from liver suggest that there are at least two enzymes, distinct from lactic dehydrogenase and from the cyclophorase-racemase system, which catalyze the oxidation of α-hydroxy acids, one specific for the l- and the other for the d-isomer.