Glyceraldehyde-3-phosphate dehydrogenase has been shown to occur in three different forms in the human adult cataractous lens: a membrane bound form (M) and at least two cytosolic isozymes: I1and I2. Similar Km's for substrate, cofactor and HAsO4, were established for each form and all three forms, to differing degree, require a reduced sulfhydryl group for maximum activity. A variety of phosphonucleosides (ATP, ADP, AMP and 3′5′cyclic AMP) as well as NADH inhibit enzyme activity. Inhibition by ATP is non-competitive whereas cyclic AMP and NADH compete for the cofactor binding site. Chloride ion stimulates and inhibits enzyme activity at low and high concentrations respectively.