Extracts of spinach, maize and barley contain an enzyme which catalyses the formation of hydrogen cyanide from glyoxylate and hydroxylamine. The enzyme is dependent upon ADP and a divalent cation such as manganese. Glyoxylicacid oxime is a poor substrate for the enzyme. Carbon dioxide is another product of the reaction and is probably produced in 1:1 stoichiometry with hydrogen cyanide. The possible relationship of this enzyme to the regulation of nitrate reduction is discussed.