The effect of some reductones on glyoxalase I prepared from animal and microbial origins has been studied. The enzyme was extracted from ox liver or baker's yeast and partially purified by ammonium sulfate fractionation, gel filtration and ion exchange chromatography. Aliphatic reductones such as ascorbic acid, ascorbic acid 3-phosphate and triose reductone showed strong to medium inhibition, while dehydroascorbic acid showed no inhibition. Kinetic analysis indicated that the inhibition mechanism of ascorbic acid was uncompetitive. Varying extents of inhibition were observed among three kinds of diphenols belonging to aromatic reductones. They were in the order of increasing inhibitory power resorcinol, hydroquinone and catechol for the ox liver enzyme, and catechol, resorcinol and hydroquinone for the yeast enzyme. p-Benzoquinone, an oxidized reductone, exhibited marked inhibition on both enzymes. Its action seemed due to reaction with amino and/or sulfhydryl functions of enzyme protein and those of glutathione, one of the substrates.