Hydrogen sulfide is highly toxic to higher animals, causing death in the same range of atmospheric concentrations as does hydrogen cyanide [I]. Unlike cyanide, however, hydrogen sulfide is sufficiently common in biological systems to pose a threat to the organism. The anaerobic metabolism of many bacterial species common to the colon and periodontal spaces, acting on proteins and other sulfurcontaining compounds, is known to liberate hydrogen sulfide [2-41. Indeed. HIS has been shown to form a small but offensive component in human flatus [5] and halitosis [6]. Direct ingestion of foods subjected to bacterial spoilage [7, 8] or of mineral sulfides dissolved in water offer other routes of exposure, as does dissolved HzS that is found in fermented beverages] 9].

Since the alimentary tract is the major portal of entry in each instance, it follows that the gut mucosa, in particular, may benefit from a mechanism for the detoxication of hydrogen sulfide. We propose this function for thiol S-methyltransferase, an enzyme present in the microsomes of many mammalian tissues [lo]. The enzyme catalyzes