Abstract: Mononuclear phagocytes appear to synthesize kynurenine-like products from the oxidation of biologically active indole compounds including melatonin, catalyzed by interferon (IFN)-γ-inducible enzyme indoleamine 2,3-dioxygenase (IDO). Concanavalin A (Con A) is a plant lectin that induces interferon-gamma (IFN-γ) production by T cells. In this study we investigated whether Con A-primed peritoneal macrophages are able to oxidize melatonin to N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK). The AFMK production was accompanied by chemiluminescence. It was found that Con A-primed but not resident macrophages produce AFMK. Surprisingly, Con A-primed macrophages from IFN-γ-deficient mice were as effective as macrophages from IFN-γ-sufficient mice in oxidizing melatonin. Moreover, addition of an inhibitor of IDO (1-methyltryptophan) did not affect melatonin oxidation. Con A-primed but not resident macrophages have a significant content of myeloperoxidase (MPO) and inhibition of MPO by azide completely blocked chemiluminescence and AFMK production. Thus, our findings provide evidence that melatonin oxidation by macrophages may occur through a mechanism dependent of MPO and independent of IFN-γ and IDO activity.