An effective competitive inhibitor of mammalian glyoxalase II has been synthesized and studied. The compound, S-carbobenzoxyglutathione, is almost totally inactive as an inhibitor of mammalian glyoxalase I. This is in marked contrast to other glyoxalase II competitive inhibitors, which in general are even more effective against glyoxalase I. S-Carbobenzoxyglutathione has found utility as an affinity ligand for the purification of rat liver glyoxalase II, and it may well have use in the study of the glyoxalase enzymes in vivo.