The characterization of human selenocysteine lyase, an enzyme that specifically catalyzes the decomposition of L-selenocysteine to L-alanine and hydrogen selenide, is described. The enzyme is the first described that acts exclusively on a selenium compound. The enzyme from human tissues, analogous to that from pig tissues and bacteria, requires pyridoxal 5-phosphate as a cofactor. L-selenocysteine is the sole substrate with a Km = 0.50 mM. L-cysteine is a noncompetitive inhibitor of the enzyme with a Ki = 5.85 mM. The following amino acids and purines are inert: L-cysteine, selenocystamine, seleno-DL-methionine, 6-selenopurine, and 6-selenoguanosine. The enzyme was found in liver, kidney, heart, adrenal and muscle in decreasing order of specific activity. The enzyme activity in liver was found not to be related to tissue selenium concentration or glutathione peroxidase activity.