Ethionine is a non-proteinogenic amino acid structurally related to methionine, with an ethyl group in place of the methyl group. Ethionine is an antimetabolite and methionine antagonist. It prevents amino acid incorporation into proteins and interferes with cellular use of adenosine triphosphate (ATP). DL-Ethionine is a racemic mixture of the anti-methylation agent L-Ethionine and its enantiomer D-Ethionine. DL-Ethionine is used as a dietary supplement to accelerate cholangiocarcinogenesis in vivo. D-Ethionine and D,L-ethionine strongly enhanced the activity of prolidase II compared with L-ethionine. DL-Ethionine is used to induce oxidative stress in liver to study the levels and activities of anti-oxidative enzymes and compounds such as glutathione. As a therapeutic agent in the treatment of malignant tumors of man, ethionine had no demonstrable efficacy. No changes in the appearance of tumor cells were noted after its administration, even when marked effects on other tissues were evident. The distressing toxicity of ethionine prevented further extension of the investigation.