Acetyl-N-formyl-5-methoxykynurenamine (AFMK) results from the oxidative cleavage of the pyrrole ring during melatonin oxidation by myeloperoxidase (MPO), a superoxide anion (O)-dependent reaction. AFMK is also expected to be formed from oxidation catalyzed by the unspecific enzyme indoleamine-2,3-dioxygenase (IDO), found in a variety of cell types including monocyte/macrophage lineages. MPO- and IDO-catalyzed melatonin oxidation has the requirement of O in common, a species formed in large amounts in inflammatory conditions. The non-enzymatic formation of AFMK can also be expected by its direct reaction with highly reactive oxygen species, such as hydroxyl radical and singlet oxygen. Thus, we assume that AFMK is a product formed in a route of melatonin metabolism, especially active in inflammation. As AFMK is biologically more active on leukocytes than melatonin, the metabolizing of melatonin to AFMK at inflammatory sites possibly plays a role in immunomodulation. AFMK is found in the CSF of patients with meningitis, and in some samples at a remarkably high concentration. AFMK was also found in some patients to exceed the concentration of melatonin normally found in serum.